I read through bossymarmalade's suggestions, and go them if they can pull it off - it sounds like a lot of fine-tuning for each of those suggestions, especially in a multifandom challenge. I'm not dissing it, it sounds like good fun, but I can see complications for each of them - even the simplest one, which is book!canon, since, say, Red Dwarf (of course I was going to say Red Dwarf :D) has book canon that retells TV canon in a different way. Both have the playing-pool-with-planets scene, but the context is different and leads to an approximation of the events in "Marooned"... So if you write a "Marooned" fic and only change the background of the situation, does that count for book!canon? Gosh.

Would I be correct in assuming that the idea of having a separate challenge for a different conception of "small fandom" arises from people wanting to write alternative big-fandom fic like the stuff you might find on where_no_woman? It occurs to me that this could be achieved by just having a minor characters challenge. You'd need fine-tuning with that as well - what counts as a character, what counts as a minor character, how big a secondary role can major characters have - but it would let you write all the big fandom alt fic you like, with focus on the sort of characters who don't usually get the limelight. It would also automatically rule out the most popular pairings.

Though, I don't know if that would make people happy either, since people generally want to write about the main characters.