Someone whose opinion I respect, and who knows I write fanfic, has the following idea:- All fanfic about anything that has been publicly printed/transmitted is legal, with the standard "I don't own these characters" disclaimer. Partly because, hell, you can't stop it anyway.

But! Any really cool idea brought up in a fanfic can be used by the owners of the original show without lawsuit. Basically his idea is that fanfic should be legal, but the foundation concept (and any spinoff idea) rests with the show's creators.

Hey, I'd buy into that. I don't write for the money (hah!) and seeing one of my ideas get picked up and made into part of an episode would be the Best Thing Ever.