Your friends can pick a relationship, a story arc, a missing scene, or pretty much anything they want, unless the author has previously mentioned that they will not write it. They comment with what they want, and you write drabbles and post it in your LiveJournal.
Fandoms (including some I've never actually written for before- EXPANDING MY HORIZONS, YO!):
FF8, 9, X/X-2
Lord of the Rings
Star Wars (canon and EU up to the start of New Jedi Order)
So... okay, shoot, although I don't promise that whatever comes out will be at all sane. /disclaimer