Just some basic ideas I put together,
Ignorance is bliss, the less you know, the more magical power you have. But remember, knowledge is power...
Elf who for whatever reason, switches over to the orc army, and fights against their own people.
Wizard who switches to dark magic to further his research, discovers council of wizards is using dark magic for evil, and decides to take matters into his own hands...
Superheroes who have a limited amount of energy they are born with, and when they use their power, part of it gets used up. When all your energy is gone, you die. Leads to lots of self sacraficial moments.
These are just some simple ideas I came up with. I have a few more, but I'm saving those for myself. *wicked grin.*