Personally I wouldn't mind if there were more ecchi games out there. I truly respect the artists for being able to draw some of the craziest poses out there. Though admittedly, the plots in ecchi games could use A LOT of fixing! But I digress....
Addressing the main topic, however, while I think it is a neat idea, it is harder to implement (someone else was discussing it earlier). I think the main issue here would be, of course, making sure that you (in some ways) and whoever you're working with, are experts on the subject you're "teaching." It would require a lot of organizing and deciding what's important to implement in the VN and what isn't (like a curriculum, if you may).
I might need more clarification on this, but how exactly do you intend to execute this VN? Will it be like a textbook type of thing where you just make decisions based on what's being taught? Or will the information be presented in the form of a story? That's another thing that would need to be decided. If you choose the former, it would be a little...redundant. You could just buy a book, special teaching software or take a class online. if you go for the latter (someone mentioned this), you need to make sure that it is interesting engaging in some way, presenting the information subtly without making it OBVIOUS that it is trying to teach you something (but then again that all depends on how the creators would go about that).
Personally, I find overtly direct messages very cheesy and annoying. For example, say you're watching a movie and the characters CLEARLY state the moral of the story and it sounds like a sermon (i.e. "You should never bully!", "you should accept yourself for who you are", etc). I think educational VN's could be unsuccessful if they come across like that, but that's just my opinion.
That being said, I think that Visual Novels work best when the topic presented is something that has to do with the social sciences, such as social issues or history, or literature. I would say these work best because they're easier to implement in the form of a story and make options that leave the player wondering about the consequences of their actions. Stories about social issues CAN be pretty effective to expose to the player the realities of such issue, like slave labor, classism or sensitive topics like rape (provided that it doesn't sound like a teacher giving you a lecture). Of course, I have not read ANY works that address these topics head on, most of the ones I've played (japanese VNs at least) treat these topics pretty lightly, particularly rape. Most VN's that are non-ecchi and center on dystopian worlds tend to focus on a more philosophical spectrum, so they're trying to prove an idea (particularly the effects of said dystopian world).
As for more science and mathematically oriented subjects...that's more difficult to implement. IMHO, science and math are very complex by nature and are not really something that a visual novel can teach (at least not without help or if the level is not very high). I'm not saying it can't be done, but I think this would need to be approached very carefully.
I think it could also work with languages...I haven't put much thought to this one. I think it could very well work
There's a ton of factors to consider when approaching this area. But I think these are the biggest issues here: Is it worth doing? Who's your audience? And WHO WOULD BE INTERESTED IN BUYING/PRODUCING/SPONSORING THIS PRODUCT? That last one is probably the most important because, at least here in United States, the general American population isn't as interested in anime/manga, much less in visual novels. If anything, there's some stigma on visual novels as they're known for being pornographic games. Luckily though, there's been more VN's introduced in android/apple products (the plots are pretty lame though....), so you could probably distribute them that way!
Anyway...those are my thoughts on educational VN's. I'm a bit of a skeptic but I'm not against the idea. Seriously though, we need to make visual novels more popular here!
