Hmmm...
Since you seem to be fine with almost whatever, I have a few recommendations (that are in no way just me typing some of my own favorites...):
G-senjou no Maou: Imagine the mind games of Death Note, but without the supernatural elements. It has a strong story, well-written characters, It also has a few VERY good plot twists. If you are familiar with classical music, you might recognize most of the BGM, since it's heavily based on famous classical pieces. On terms of Lighthearted/serious, it lies mostly towards the serious side.
Hoshizora no memora: A story about a promise... (and also about a few more events). It has no particular "strong point", but is consistently great in terms of story, music, and characterizations. The artwork is also very enjoyable.
I chose two that are around "middle length", so if you'd like you could try them out.
I also recommed Kaguya's list 50 translated VNs you should read before you die