My fanfic policy is: Go for it. I’m honored you want to play in my world. I won’t read what you write, so please don’t point me to it–this is partly because of legal concerns, but also because, knowing how I write, I fear reading fanfic set in one of my worlds would interfere with my own creative process, which involves pretty deep immersion in my vision of those worlds. And also because I can’t imagine anything that would throw cold water on the process of writing fanfic as quickly as knowing the author was watching over your shoulder. But please don’t take my lack of involvement as disapproval, because it’s actually quite the opposite: the thought of fanfic written in my worlds honestly delights me. (Just don’t go out and make any money off of it. Because neither of us wants to deal with the legal issues that would bring up.)
And that’s pretty much it, at least at this point. But for anyone who might be interested, here’s my own fanficcing background, which has informed my feelings about writing it.
My writing began with fanfiction, though I never posted it in public. This was in the days when only academics had Internet access and few people owned their own computers, so I didn’t even know it was called fanfic, or that anyone but me and a few friends wrote the stuff. We worked hard to keep what we were doing secret, convinced that we would pretty much die of embarrassment if anyone knew. (I wonder, now, how many classmates were secretly doing the same thing.) Fandoms I played in included Battle of the Planets, Star Wars, Voyagers! and the Wrinkle in Time universe. (You might think it’s impossible to write Wrinkle in Time/Battle of the Planets crossover fic. You’d be wrong.) Writing these stories not only gave me a place to hone my writing skills–it also gave me a place to escape–and to play, long past the age where pretend games became otherwise unacceptable.
My first short story sale came out in Marion Zimmer Bradley’s Leroni of Darkover. I didn’t think of it as fanfic at the time, but of course it was. (I do think there are differences between writing a story for an anthology edited by the creator of the world and writing such a story only for oneself–but there’s common ground, too.) I’ve since also published short stories in three Valdemar anthologies, and my latest novel is ultimately Icelandic saga fanfic. (The sagas being, of course, very much in the public domain.)
All of which is part of why I won’t get in the way of any fanfic you choose to write, not so long as I have a choice–and as far as I can tell right now, I do.