I can't think of a single game that did this sort of thing where it did more good than harm. Maybe Morrowind.
The closest thing I can think of is "rank" systems in shoot 'em ups. Essentially, the game's difficulty increases as you collect items, destroy enemies, and survive, and it lowers if you take damage or avoid items. It'd be easy enough to implement in an RPG - just give the enemies level ups based on the player's rank. You'd need to decide whether you want the player to be able to manipulate their rank to get better results. I hear IVAN is like that, actually.
Random bosses could work, but you'd lose the ability to make interesting, hand-crafted encounters, and you'd need to be careful to prevent the game from generating overpowered or underpowered combinations of traits. You don't want to be like ToME 4 where random bosses can sometimes do 20 million damage by looking at you.