I was kicking around an idea about higher-level Magic-Users getting obsessed with some theoretical magical concept. You know: the crazed wizard who gets an idea to cross-breed fantastic beasts or create a new kind of enchanted servant, or create a seemingly useless spell. After all, look at all the weird monsters that are supposed to be the product of magical experimentation. To a certain extent, any player who picks Magic-User as a character class is probably going to have some crazy magical obsessions already. But what about those useless experiments? How can you encourage that kind of craziness?
I hate the idea of taking over a player character, so that's off the table. But then I thought: why not just say that obsessed characters can't rest while avoiding their obsession? In other words, no healing unless you've indulged your obsessions recently.