When you're stumped for story ideas, fan fiction isn't such a bad idea. The characters are borrowed, and so are some of their goals, motivations and conflicts, but those things change all over the place when a fan fiction peice is created. but fan fiction is also a great tool when you're working on developing plot. Character is already set up and roaring to go, and you don't have to worry about it as much.
If you have issues with telling a good story (creating a good plot) why not play around with some fan fiction? Once you've got the hang of things, throw in your own characters and have a ball.
Go for it - just remember that you can't publish fanfiction because you don't own the rights to the characters.