I can't see the appeal of fan fiction for writers. If you want to write, write. And that means, create your own characters.
I agree with Chelsea Quinn Yabro, who's quoted on Lee Goldberg's blog: "If fans want to write, they should make up their own stories with their own characters. That's what fiction is all about."
There seems to be some kind of fiction (play on words intended) among some in the writing community that fan fiction is a "grey area". I can't see it. If you use someone else's characters in something you write, make sure the work is in the public domain. If you're using characters created by a writer who's alive, or whose work is still under copyright, you're stealing someone's livelihood -- either that of the writer or his heirs.
And there's no "But..." about it either. It's theft.
OTOH, if you want to get permission to use the work, then apply to the author/ publisher, and be prepared to pay a permissions fee.
A lot of fan fictioneers' pouting responses to Cease and Desist requests from writers is along the lines of: you take yourselves too seriously, we're FANS by heck, and we can do whatever we want with your creation.
Yes, you can, in the privacy of your own home and your own computer. But if you publish the material -- and putting it online is publication -- prepare to be sued.