It occurs to me that there's an irony to all the controversy around fanfiction. You can't write a story set in someone else's universe. All you can do is base your setting on your experience of someone else's setting. And infringe their trademarks, of course.

I suppose you could make a case that the worlds of, say, Star Wars and Star Trek are already so massively collaborative that there's no necessary real difference between Paramount's Star Trek and yours. But for, say, Middle Earth, for all the reams of supporting material we have, there's still a vision, a mood, that was in Tolkien's head that informed every sentence he wrote, and that can never be perfectly recreated.