would be cool if in a route or something you get turned into like, a demon. I just quite like TFs like that, where you get turned into something not human, or something like that. just an idea.
A Succubus transformation would be possible near the end of the game. There is a cow-girl transformation in the second dungeon, if you were unaware. I always make those kinds of transformations temporary though, it's too much work to have them factor in to all of the scenes in the game.