I always have problems with fan fiction. This is partly because it actually forgoes the opportunity to create your own world. Also it means adding to somebody else's franchise for free and if what you produce is good they can just take it away from you thanks to copyright law.
I would say if you are excited about it then write it. If you want to write it there is people out there who want to read it. Besides anything you write makes you a better writer so the effort is never wasted.