Stories such as Warbreaker and The Runelords where magic is worked as a 'theft' of someone else's essence can also lead to interesting wrinkles, moral dilemmas and difficult choices.

I've always been fascinated by some aspects of magic with consequence. I am working on a short story where certain people gain access to a form of magic when they get extremely ill. The sicker the person the more powerful their potential magic. The more those people use their power, the more their illness defeats their bodies.