Some of the 'ships here seem underrepresented at this particular community, while others, I've heard their fans lament the fact that there just isn't much out there for their particular 'ship at all. I was thinking that a set of round robins might be a fun way to A.) get people talking about their favorite 'ships or 'ships they want to see more of, and B.) use teamwork to help out those who might be struggling with their muse. Also, it might help in terms of helping everyone to find more like-minded people. Forums are a vicious circle: people don't participate if there doesn't look like something's going on, but there isn't anything going on if the people don't participate. This might be a great way to generate some buzz about your favorite pairing/more-ing and get more people playing in your particular sandbox. Also, the people who've already been here for a while may be inclined to try something new -- which has been known to happen!
Basically, I guess I"m wondering if this is something you may be interested in. Naturally, there would have to be some kind of structure and ground rules put into place: is each person responsible for a paragraph? A whole chapter? Or do they write as much or as little as they have until they run out of steam? And if it's the latter, I suggest a "no one-liners" rule -- we did a round robin thing at a Han/Leia community years ago, and every time things started to heat up in the romance department, one particular member would swoop in and thwart it with one sentence. It was funny the first time, but by the sixth it got old.
And that brings to mind other questions: would you want established turn-taking? Or just whoever wants to picks up the ball and runs with it? My suggestion, for what it's worth, would be the turn taking, so no one gets left out or accidentally duplicates someone else's efforts, but whatever people want to do is fine. If you're worried about it being your turn and you can't do it because of a silent muse or big family thing going on or whatever, you can always say "pass".
Another thing: if you want the freedom to make it NC-17, each group will have to negotiate among themselves how they want to handle that. Not all fans of a 'ship are comfortable writing/reading it, so each group would need establish its own ground rules and protocols for that.
Also, I would suggest three as a minimum number to sign up for each 'ship. That way if someone has to pass or drop out completely, one person isn't left holding the bag.
Sorry this got long -- as the title says, I'm thinking aloud a bit!