One of the cardinal mistakes of most VN's is the severe lack of support characters and different locations, because that automatically limits the scope of the story a lot. There's just so much you can do with only character interactions between a moe blob. This is particularly puzzling since this shouldn't even be a financial problem. Usually sprites require much less effort than full CG's, especially if they aren't main characters. They also don't even need to bother their main artist with it, since they can be done by assistant artists. Similar situation for the backgrounds.
You don't really need a whole lot of event CG's for a story, just a good bunch of support characters and locations. If I want to know about the story scope of a VN, it's usually enough to just check out the character section. If the character count is less than 10, there's usually not more in it than a slice of life moege. Exceptions exist like the 9-nine series, which uses their main cast very efficiently, but those are pretty rare.