.-. well, yeah, not something really important for discussing, but Let's talk a little about this.
1. Old/Common Style - Clannad
For Clannad Opening itself, This one has good Song, thanks to my personal taste, I like Jun Maeda and pals Song (Riya(Lia), Chata, Tada Aoi and Rita).
Old/Common style has typical order -introduce their character, -input some minor CG and -Good song as support, yeah, something you will expected from image of visual novel . Some old VN use this Method for their opening.
2. Full Animation (2D) - Supipara #Chapter 01
Another Variation Of Opening we got here, compered to Common VN opening which only give A Little touch of animation, this Opening Video Has Full Animation, More like Anime Opening To be honest (as expected from minori ).
3. 3D animation (Without Vocal Song) - Katawa Shoujo
I'm not really sure this one count as "VN Opening" but I will insert this. Something Rare for me, yeah, I found something surprising from katawa shoujo. Not really well make 3D animation but you can say this something can make you impressed (again, Personal Opinion)
Unfortunately, This opening not have any Vocal Song, just Instrument music and have Short duration.
4. Image Manipulation - Wagamama High Spec
This type of opening use their CG & Sprites and mix n' remake them into something Really damn Good, and included some special effects and well make touch of Animation. Most of VN these day use this method.
5. 2D or 3D ? Mix Them All - If my heart Had Wings
This one has mixture of 2D and 3D, Use CG and also 3D animation. Include some Special Effects, and mix them all into well make Video.
I think that's enough, If you know something different , Tell about it to us. and Give me Your Favorite VN opening.