All I'd ask is that it not be ONLY drabbles. Because I don't get good fic inspiration that often, and when I do it is not always something that will fit a drabble.
I'm thinking that it would be open from drabble to however long you wanted. Heck, I'm even thinking that if someone wanted to write fannish non-fiction (a look at ideas of redemption, etc. in "Firefly") or a spec script, why not?
Will we get to pic our fics? Because it does me no good to tell me to write Grey's Anatomy fic, or something.
I'd probably suggest that people list up to five different prompts in five different fandoms. I think that would increase the chances of being able to match requests to writers (who would, of course, list what they're willing to write).