Anyway, once you've got your hero or heroine in mind, make them evil and then write them into a story. What happens when the good guys turn to the dark side? Who will stop them?
If writing a novel from scratch is like writing a symphony, then writing fan fiction is like improvising a jazz solo: You can't change the underlying structure or the main melody, but you still have a lot of room to play around in.
There isn't a lot of space to make a living writing fan fiction, and the legalities of even trying to sell it give me the howling fantods. But still, playing around in some other writer's universe can be a good writing exercise.