I'm think 5 teams, each with a different theme/genre (or a different genre/common theme?). You can contribute anything you want to whichever project, but you can only work on one of the five. At the end, everybody votes on one of the projects (if you're competing you can vote, but only for one you aren't working on), and the most popular project wins.
It's kind of an anti-Indie challenge, a challenge to focus the most on what you do best, a challenge to trust others to do the same, and a challenge to comment/communicate enough to function as part of a team. Each project gets a subforum, and you must join that project's "group" to participate in the subforum (via a "Sign me up for..." sticky for each of the projects). At the end of the compo, the subforums are unlocked for all to see; like a post-game "making of."
Finally, each project makes a thread on the main forum where any contributors can do their own post-mortems, or revisions to it. Maybe a 2-week time frame for setting up the first/main teams and establishing the projects, and a 2-week time frame to make the project? (Bearing in mind, nobody has to build alone from the ground up, here.) You could possibly cut each phase down to one week apiece, even.
PLEASE MAKE COMPO LIKE THIS!!