I had a neat idea for a video game mechanic. Make a character with a mana bar that doesn't recharge. Basically, your character can magic away any problem, you can change someone's mind, you can defeat any monster, you can steal any item, but it's all done with a finite pool of energy.
This has a large problem, most players would be too afraid to use their powers, since it's likely it'd be needed more in the future. Many players keep their most powerful abilities in reserve because they might be needed later, this would be even worse when your main power can do anything and can't be refilled.
So what's the benefit? It lets you skip parts of the game you don't like, with an in-game excuse. If you don't like fighting a certain enemy, or if you don't like fighting at all, you can make it much easier. It could also make for an interesting conflict in the character, since he's sort of infinitely powerful, but the character knows they can't just use it on everything, or they'd be helpless. This could either lead to a character that lives on bravado, or one that is deeply insecure.