Clephas

With JVNs, in particular untranslated ones, saves are intricately intertwined with how people play the game, with save points being specifically recommended in walkthroughs to shorten replay time when moving to other routes or filling out CGs and scenes.  A lot of people who make walkthroughs for translated games in English have a tendency not to transfer this system over, even if they used a Japanese walkthrough as a base, a lack of convenience I found notable, even if I honestly don't know why.
If you want to look for an ideal form of this kind of walkthrough, Foolmaker (gusha no Yakata) is still preserved at the Sagaoz website (along with game saves for a lot of old VNs) and contains a bunch of efficiency walkthroughs that save at the latest point possible for each path split, which is why I generally recommend it for people playing old JVNs.
Seiya-Saiga, which is the oldest surviving walkthrough producer for visual novels in Japanese, still releases walkthroughs for all the releases in a given month.  They also place save points at key points in progression to make things easier for readers who have less time and don't want to have to figure things out themselves.
 

The biggest hazard with the ladder-style story progression structure is that all the heroines but the 'true' heroine get gypped.  Aoi Tori, at least, has the advantage that all the paths are technically 'true' in an alternate worlds sense, but it still feels like the heroines other than Akari aren't really getting treated equally.  
In the case of Aiyoku no Eustia, the author failed utterly to create internal consistency, as his setting (both the general one and the heroines' settings) made any path other than Tia's path impossible within the greater context of the story.  This is actually a trend with a lot of writers who like to use the ladder-style, where they fail to maintain internal consistency due to a half-assed attitude toward the non-true heroines' stories.  

The apocalypse genre is greatly influenced by the post-war (WWII) mentality.  In post-war Japan, which rebuilt after seeing most of their major cities firebombed and their industry destroyed, the concept of an apocalypse was much closer to reality, not the least because of the nuclear bombings.  Similarly, much of Europe's urban landscape was a ruin due to the bombings from both sides, and rebuilding afterward was a colossal undertaking that made it easy for the Soviet Union to take over a massive portion of the continent like it did, through means both forceful and more subtle.  
As a result, most apocalypse genre works in all mediums are descended from the ones inspired by this era and the threat of nuclear war that persisted through much of the twentieth century afterward.  For a generation that had witnessed the disintegration of two major cities and now had multiple hostile powers possessing similar - even more powerful - weapons, it was much, much easier to imagine the annihilation of humanity than it was for previous generations.
Most current works are evolutions on the same ideas as those from that era.  There is a sense of cynicism about human nature, a sense of helplessness when it comes to an individual's ability to effect outcomes, and a sense of desperation for survival that influences preppers and all other sorts of people even today.
I personally would like to offer the most unique variation on the post-apocalypse experience I've seen in a VN (or rather the most modern one).  Komorebi no Nostalgica.
This VN is unique in that, at first, it is difficult to tell that this is a post-apocalypse landscape.  The characters are living normal, peaceful lives in a seemingly super high-tech world, but when they discover an ancient android, the predecessor to the sentient AI androids called the Metosera that now live as independent sapients amongst them, the perspective changes drastically.  While the basic atmosphere remains soft and disconnected from what came before, the characters are slowly confronted with a line of thought that predates their current society and the remnants of the old war.
Another example of an interesting apocalypse story would be Evolimit, one of my all-time favorite VNs and written by the same author as Tokyo Babel.  I'm going to spoiler this one as well, since I intend to just talk openly about various aspects of the setting and story.
Baldr Sky (all of the series) is a perfect example of the post-apocalypse and apocalypse genres.
 

The Japanese dating sim genre hit is height back in the late 1990's and early 2000's, before vanishing almost entirely with the advent of modern visual novels.  In most ancient dating sims, you would spend time polishing yourself (studying, working out, etc) on some turns while doing part-time jobs to earn money for presents and conversing with heroines (often repeating the same set of questions and interactions over and over) on a regular basis until you could get their affections high enough they were willing to go on a date.  Depending on your choices during the date, you would get closer to an actual relationship, which was the end goal of most dating sims (since most of them weren't h-games).  
Dating sim elements were leftover in most SOL VNs during the mid and late 2000's, but by 2010, most dating sim elements (even hidden ones such as affection points) were completely eliminated from modern visual novels.  
As a side-note, references to dating sims persist in anime and manga long after the Japanese version of the genre died, and they made a comeback with Persona 3-5's social links system.

This game fixes most of the quality of life issues the original game had, while creating an interesting atmosphere typical of Japanese horror.  The death ends weren't as abrupt as in the original game, as Pallas says.  They are pretty telegraphed, and by the end, I was mechanically saving right before key points and seeing the dead ends before moving on to the main story.
I disagree primarily with the assessment of the rpg system, but that is because I honestly see nothing wrong with 'classic' rpg combat systems in general.  I'm generally playing jrpgs while I'm half-asleep after work, so I don't want to be troubled by things like real-time battles or complex gimmicks that aren't really a 'quality of life' improvement for an old-style solo gamer like me.  To my mind, the turn-based battle system was perfected with Grandia II, and I think all jrpgs should use it, lol.
As a visual novel, sadly, I've read many, many better ones, even ones in the same genre.  Which is kinda sad, considering that Compile Heart put more money into the game than any five of those VNs combined (with the possible exception of Minori, who was horrible about managing their funding, or Key who always blew money on mini-games and silly animations with their later games).  
Edit: One thing I should note is that, as visual novels and visual novel hybrids became more ubiquitous, telegraphing bad ends became more and more common, even in the horror and mystery genres.  Back with Tsukihime, the bad endings (really just dead endings) were not predictable to any significant degree (the original Tsukihime of course), and most of the visual novels made from 2000-2012 used some variation of a koukando (love point/affection point) system to determine which heroine route and which ending you ended up with, if they weren't using a strict 'choose your own adventure' format.  It is very rare to find a mystery or horror VN made before JVNs began to decline five years ago and after 2015 that wasn't fairly obvious about which choices led to dead ends or bad endings if you were paying attention. 
The major reason most people end up using walkthroughs for VNs is because it takes way too much time and effort to figure it out on your own, compared to rpgs with multiple endings and the like.  Sometimes, VN-makers like to create triggers for bad endings or 'normal' endings in the weirdest places, even going so far as to erase affection point gains simply because you selected the wrong choice in a joke scene (real story).  
 

It isn't really an issue if you can read a VN quickly enough, but I can see how the skip function might be an engrossment-breaking tool for the majority of people, who can't read a line a second, lol.

Science Fantasy is perhaps one of the most pervasive genres in existence.  The most powerful example in Western culture would be Star Wars, which is often considered the penultimate science-fantasy by fanboys.  However, the genre also gained some minor popularity elsewhere, with the Deathstalker Saga and a few others prior to the current fantasy/sci-fi publishing boom that hit after ebooks took off.  The Superhero genre can also be considered to be science-fantasy in a general sense, making it a bigger umbrella than most think.
JRPGs such as the old Breath of Fire series and Xenogears both included elements of science-fantasy, mixing magic and tech freely, and this was also seen in Final Fantasy VI, where magitech was at the center of the story for much of the game.  For VNs, my beloved Silverio series is worth mentioning, as the series has a basis in science-fantasy rather than being pure fantasy.   
Perhaps the most notable aspect of science-fantasy is that it combines the flexibility of fantasy with an internal consistency reminiscent of science-fiction in many cases.  Fantasy is an infinitely flexible genre with no real limits, whereas science-fiction is usually an extension of our world, seen as a potential future or past.  When the two are combined, you get a genre that seems partially like an extension of the world we know and partially a realm of pure fantasy and imagination.
 

This game, in retrospect, didn't really do justice to any of the heroines other than Shinku.  I can't say I recall any of the paths or the details of the characters anymore, whereas I can recall most of the details of Sakura, Moyu and even AstralAir by the same company.  The incidents in the other worlds left a much stronger impression than most of the heroine routes, which is perhaps on purpose.  
Favorite's specialty as a company is playing on emotions, and the other world chapters and Shinku's path do an excellent job with this, while the other paths are mostly just background information to add depth to Shinku's path.  While this was true - to an extent - with most of Favorite's other games (feeding the true path with the other heroine paths being a common tactic when there is a true path), this one is perhaps the most obvious about it.  Nonetheless, it is a game that is worth mentioning for people who want the catharsis of emotion brought forth by the characters' situations.

A few examples of multiple Title Screen styles.
Evolimit: Depending on which ending was the most recent, the final background CG from that ending is visible on the title screen, while that ending's unique BGM is played.
A number of VNs: The title screen starts out blank, but as you complete heroine routes, the heroines are added to the title screen in various poses until the scene on the front of the game box is complete.  This is the most common style, seen in numerous VNs across many genres.
A number of VNs: Background change (usually along with BGM music).  This one was common in the 'early era' from 2000-2009.
The title screen change is most common in plot-heavy VNs, whereas it is rare with mid-budget charage/moege (your basic sol-romance).  The main reason for this is that those type of games tend to pour most of their budget into the h-cgs, heroine sprites, and scene cgs.

A lot of the same people who enjoy otomege also enjoy Yaoi-ge, and for obvious reasons.  A disproportionate number of the heroes in yaoi-ge are the forceful type that is plentiful in darker otomege that have 'love-hate' paths and yandere paths.  Tbh, this sub-genre is something I'm only familiar with because my cousin kept trying to get me hooked on it, and I refrained from reviewing them or rating them on vndb in order to avoid excessive bias (for similar reasons, I only rated a few of the otomege I played).  To be blunt, at least in the ones I played, it seemed like there was a large percentage of paths that either went from bromance to romance or were were the result of some kind of insane obsession on the part of the hero.  Given my own predilections, it was just disturbing to me, and I thus refrained from rating them, because I DEFINITELY am not the audience these games are meant for.  There were a few times where the protagonist was the forceful one, but most of the time the protagonist was just a cipher or a doormat.  

SRPG hybrids can be a mixed bag.  Classic style (The Shining Force/FFT/Fire Emblem type) that use stages are ideal for matching progression with participation.  However, in my experience, the more the balance goes toward strategy, the more likely the story starts to fall by the wayside.
Venus Blood also strikes a decent balance between the two sides, using more standard strategy (based on building army units that can counter enemy advantages and build on one's own, while picking how one expands one's field of influence and other aspects).  It is perhaps the most gameplay-leaning series that doesn't fall into the trap of over-focusing on the gameplay to the detriment of storytelling (which is the entire point of a VN).
Where things start to tip in the other direction is games like the Sengoku Hime series and Madou Koukaku.  The former is a series where you fight to conquer Japan using feminized Sengoku Jidai units, and it is very heavily tilted toward gameplay throughout most of the series, except in V, where progression and storytelling was more tightly scripted (The Oda plotline and Ashikaga one in particular were really well-designed to bring the characters and their trials to life).  Madou Koukaku goes a little too far in the direction of gameplay at times, in particular if you don't choose the 'true history' or 'magic' routes.  Those two routes have a somewhat scripted storyline that you can follow, but if you just go around conquering, the game quickly becomes a slog devoid of story other than the collapse of your enemies and your protagonist's corruption due to power lust (which isn't necessarily boring, but it is significantly less interesting than the more scripted stories of the two primary routes).
Overall, all gameplay hybrids need to strike a balance between the gameplay and storytelling, and in my experience, it is those that keep the focus on the story that turn out the best.

In my experience, VN dark fantasy either emphasizes an endless path to despair or a path through despair to hope for the future.  Nitroplus tends to focus on the former, as is seen in Tokyo Necro and Muramasa, where the majority of paths and their endings are unrelievedly dark from beginning to end.   Indeed, there is this belief in a lot of Japanese makers that the dark needs to continue right through the epilogue.  When there is an exception, it often comes with a backlash.  
One particular scenario that annoys me to this day is the Ein route vs the anime of Phantom of Inferno.  In the original Ein route, the ending has Ein smiling and happy for the first time in her life, whereas the anime has the protagonist murdered during their trip to Mongolia.  This contrast seriously pissed me off, because Ein's path was essentially the 'good ending' of the game, the only path where there was some level of salvation, and the anime makers ruined it out of a desire to turn it into some kind of twisted moral lesson.
Anyway, one thing I find ironic is that there is far more dark fantasy on the otome-ge side of things than there is on the male-oriented side of things.  I'm sometimes stunned at how deep into the darkness the otome genre goes, with levels of emotional brutality (both toward the characters and toward the reader) that are frightening.  I mean, I like yandere heroines, but yandere heroes are freakin' scary.
Dark Fantasy in male-oriented JVNs hit its peak in the mid-2000's, and most of the games were focused on extreme stuff similar to Maggots Baits, even if it went in a different direction.  At present, the only real (non-nukige) dark fantasy company still operating is Clock-up, since Nitroplus hasn't put anything new out in half a decade.
Edit: Incidentally, to this day I have extremely mixed feelings about Maggot Baits.  It had excellent action and if you cut out the H, the story was great.   However, if you include the H, it all becomes disgusting smut.  

Oh, you also mention characters acting oddly to make the story progress, but this is fairly common in plot-focused VNs.  In early VNs, it was mostly because of the philosophy of 'gotsugoushugi', which is a game-maker philosophy for plot-focused games with multiple routes where character actions and even personalities are warped at a moment's notice solely to move the story forward.  The real issue is a matter of degree.  For instance, there are a number of VNs I can think of off the top of my head where the personality change of a main character was so dramatic it completely broke me out of my engrossment in the story.  The most egregious example of this I can name right here and now is Tsuki ni Yorisou Otome no Sahou 2.  During the common route of the game, the protagonist is an arrogant, immensely capable person with a wide variety of talents, but in each of the heroine routes, his personality gets toned down, his talents weakened, and his motivations twisted in ways that don't make sense even in the context of the route.  This is an example of going too far with that kind of tactic.
More subtle are the random bad endings that pop up in a lot of horror and action VNs (especially early ones) where the actual choices result in random horrible deaths that make absolutely no sense in the chronology of the story.  These games were the reason I started resulting to using walkthroughs for all VNs, because I got tired of making choices, only to end up with senseless bad ends or normal just friends endings.  However, at the same time, this degree of manipulation can actually result in a positive result, as while those endings were often frustrating, they were also part of the experience as a whole and made it richer in some way.
To be honest, it is one of those JVN storytelling techniques that can be either excellent or horrible depending on how well it is utilized.  

This isn't the only VN series that organically turned into a trilogy or a duology.  A perfect example is Akatsuki no Goei.  It has an almost identical approach to continuing things to Grisaia, in that the second game is a combination of after-stories and a route that covers the protagonist's past (which is incidentally even more brutal than Yuuji's).  The big difference is in the third game, which is based off of the Kaoru route of the first game (non-romantic route).  While it has a core story, it has three main routes and numerous heroines, the big change mostly being in what perspective the protagonist sees the story from.  
As an approach, it has its positives and negatives.  Kinugasa Shougo, the writer for Akatsuki no Goei, hates conclusive endings, and it is pretty evident that Shangri-la (later absorbed by Akabei) forced him to conclusively end the main series in Tsumibukaki Shuumatsuron.  His resentment came out in spreading the reader's attention across way too many heroines, in my opinion.  
Grisaia came out later, so I always figured that they learned from the excesses of the Akagoei trilogy when they made Grisaia into a trilogy, focusing on the cast of characters rather than expanding it massively.

I never liked timed choices as a mechanism in VNs... most of the time, they were just annoying.  I guess it is because if I wanted to feel pressured, I would just go play a normal video game like a shooter or an a Souls-like game.  

To my mind, space sci-fi represents the optimism for the future that has yet to be repressed by people's increasingly negative outlook on our civilization's future.  They are futures full of exploration, danger, and wonder, with cultures both different and the same.  While many space sci-fi are dark in atmosphere, that doesn't get around the fact that even in the darkest space science-fiction, there are wonders and horrors that stimulate the mind and the spirit.
I usually don't make distinctions between types of future sci-fi (as opposed to modern day sci-fi), incidentally.
Space sci-fi is relatively rare in JVNs, which are my specialty.  One that comes to mind is Fake Azure Arcology, where people were forced to flee from a massive storm of asteroids that devastated Earth's surface, forming unique civilizations within the arcologies they fled to.  In a mix of high technology and restrictive lifestyle, it describes the unchanging human condition, even in circumstances that are drastically different.
Komorebi no Nostalgica, one of my favorite sci-fi VNs of all time, is based in a world where humans warred against their AI slaves until the slaves won freedom, at the cost of taking the bullet for humanity destroying its own history for the convenience of politicians.  It is a world where, interestingly, history is mostly lost to the younger generation, with the older generation only having faded memories of what was destroyed.  In this game, society has almost become a blank slate, and the game extensively explores the troubles and possibilities of human-like AI in the two different types - the ancient android Cinema,
, and the victorious Metocell, whose emotional emulation evolves over time and experience.  
More familiar to English-speaking readers is the dystopian worldscape of the Baldr series, where people have practically fled into the virtual world to escape the limitations of the physical.  While the Baldr series is a fun read, it is also the most cynical and pessimistic approach to sci-fi I've seen in VNs.  It shows the nastiness of human nature, the poor decision making of those with power, and the inevitable failures of a society that lost its way long ago again and again.  In the Baldr series, AI alternates between 'ghosts' of the deceased, god-like beings that are omniscient in their own way, and monstrous amalgams that blindly follow their programming with only a surface imitation of flexibility and thought.
Sci-fi can be a wonder or horror, but it is always looking at the future.
 

Polyamory, whether it is harem or multi-directional, is a difficult issue for people raised in the modern era in first-world countries.  While it is beginning to be normalized in some countries, it has been demonized for so long that it will probably be a long time before the stigma associated with it fades away, if ever.
In VNs, I think the one that takes polyamory to its ideal end is Draculius, one of my favorite VNs.  While Jun, the protagonist, is central to things, each of the heroines has a defined role within the family that can't be played by any of the other girls.  Belche is the mother, Xeno is the protector, Rika is the attacker, and Lian is the representative.  
I do agree that in most cases a harem should stop at three (one primary and two secondaries with a relationship aside from that possessed by the primary).  To be blunt, there is only so much time one person can put into other people, and expanding to the ridiculous size some VN harems have is just undoable.  
Traditionally, there were any number of reasons for a harem to exist.  One was in the case of royals and high nobles, where numerous heirs were necessary to solidify power and ensure passage down to the next generation.  Then there were rich merchants and leaders of settlements.  In both cases, this prevented too much wealth being permanently settled into one bloodline and reduced resentment from the population at large.  It also provides a larger supporting family for the raising of children, reducing individual burdens for child-rearing and making things more flexible without going outside the family group.
Generally speaking, early human civilizations didn't place emphasis on and in some cases didn't even have the concept of romance.  That is why some cultures retain arranged marriages and defacto polyamorous relationship setups.  (for JVN fans, the example would be the way it is tacitly understood that powerful Japanese men - like high-ranking politicians and corporate leaders - will have lovers outside of their wives).  

In my experience, JVNs - at the very least - tend to be reliant on sit-com and manzai (someone saying or doing something silly and the other person pointing it out in an amusing way).  The other common one is plays on language (kanji puns, misusing proverbs, etc) that don't translate into English and are thus usually missed or fall flat when translated.  The vast majority of comedy in JVNs is sit-com, where the situation and its inherent absurdity provides the humor or the reactions of the characters do so.  Japanese VN writers love tongue-in-cheek, though every once in a while slapstick will be dropped in to change the pace (slapstick was a lot more common twenty to thirty years ago in anime and manga than it is today, whereas sitcom was relatively rare... manzai is about the same, since it seems to be the undying genre of comedy in Japan, for whatever reason).

Some companies utilize the rigid playing order well, but others do so poorly, seeking to hide lower path quality by forcing you to experience the lesser paths first.  A classic example of this is Rui wa Tomo o Yobu as well as Comyu.  In the former case, Rui's path is by far the weakest path in the game, and yet it is the first one you are forced to experience.  The same goes for Benio's path in Comyu.   This has a huge disadvantage, as it gives an impression that the game is lower quality than it actually is and leads to people dropping it without seeing the best parts.   Ruitomo is still a top-quality game, but more than one person who has tried it at my suggestion has complained about Rui's path being so weak before dropping it.
On the other hand, Dies Irae does the same by enforcing that you play the game in the order of either Kasumi or Kei first, then Marie, than Rea.  This actually works out ideally (especially since it links directly into the Shinzabansho canon), and it also allows you to ignore one or the other if you wish (Kasumi gets ignored the most often, incidentally, lol).
The most common use of Rigid Playing Order happens in chuunige as a genre.  This is probably because the scenario directors and writers in these cases are far more likely to have a specific pathway to the finale than other genres.  The methodology I came up with is to split heroines/paths into four tiers.  I call these tiers the 'introductory path', the 'sub path', the 'main path' and the 'conclusion path'. 
Introductory paths serve more as setting and character dynamic introductions more than anything else.  They usually assiduously avoid presenting the 'hidden truths' of the setting and plot, while providing a truncated ending that deliberately falls short of what the reader would want.  The aforementioned Benio and Rui paths, as well as the Anna path in Vermilion Bind of Blood all fall into this category.
Sub paths are a bit of an odd man out.  For some reason, some VNs throw in random heroines and paths that have no real meaning for the story as a whole.  Ayaya's path in Comyu and the 'normal paths' that sometimes appear in various games fall into this category.
Main paths are the most common type of path in plot-centric VNs.  These paths have the quality of being conclusive when taken by themselves but do not resolve the plot's core conflict in a conclusive manner.  Examples of such paths include Kei's path in Dies Irae, as well as most of the paths in Tasogare no Sinsemilla. 
Conclusion paths are often represented as a true route, grand route, canon route, etc.  The universal aspect of these paths is that they all resolve the core conflict of the plot in a conclusive manner, as well as tear the veil concealing most of the mysteries hidden in the other paths.  They will often include variants of events that occurred in other paths that lead up to a grand conclusion, and there are often aspects of the plot that only make sense because you played the main paths prior to playing this type of path.

Yet another company escapes the evil grasp of social media giants.  

I should note that there is very, very little description/narration in this story.  It is almost completely dialogue based, so in that sense it falls behind most other VNs.  However, since the entire point of the VN is making you laugh, it doesn't matter as much as it would in a different VN (even a charage).  I remember that this VN made me smile on numerous occasions, so I'm not going to go any further in saying anything negative about it.
 

Sakura Moyu is Favorite's greatest masterpiece.  I recommend that anyone who can read Japanese well-enough to play VNs untranslated play it.

I played Tonoko's route first of the three the first time I played the game.  Admittedly, the route is slower-paced than either Miyabi or Shino's routes, but it has its own charm.  On my second playthrough I found Shino's route to be the best of the three, a route that got tastier on the second playthrough.  Miyabi's is a good route no matter how many times you play it.  Playing Tonoko's route the second time was not as good as the first, but it was still a good experience.  
The route that I had the most trouble playing the second time was Yuuna's, because all the shocking revelations were already over with.  It's the kind of route that is godly the first time you play it, but has little to offer on a second playthrough, in my opinion. 

On another note, I love wunderwaffels.  I just also enjoy stories where there is no wunderwaffel.  I'm an isekai junkie in general... though it is a huge letdown when a real action story that is serious has a protagonist who is too overpowered for there to even be a struggle or strife.  It's one thing if he ends up that way towards the end (like with Hajime in Arifureta Shokugyou), but most attempts to do serious plots and action with an op protagonist tend to fall apart near the end.  The exception is when the main antagonist is just as op as the protagonist.

I went for Sakura, Moyu, personally.