How about an RPG in chapters... All vaguely related, but not entirely, before actually being woven together after a few have come out. Perhaps one with each of, say, as many characters as people can be bothered to do it at some point. But they all end up at the same point.
Then there's an agreement between different authors to have their characters meet/colaborate in different ways, so 3 people who've done first chapters might then get together to work on a second, with 3 chars, and a more involved story.
And then possibly have moving of characters from different strands and such - like writing them out of one thing and into another for some reason...
Possibly the most important thing for such a project, which would be both massive *and* small (massive overall, but small in its inception), would be a constant world setting, and a general consensus not to have characters getting involved in conflicting evil organisations (or, for that matter, rebel ones).
What'd be even more interesting might be to pit some characters against one another in different chapters (say once story crosses another wth the characters in that being on the opposite side to the other ones or some such). We could end up with a really nice set of small RPGs, ideally with compatible saves (of the "just rename" variety) which gradually worked up into a frenzied complex roleplaying opera.
Once I've got some gfx done (which I'll probably get by, well, waiting for others to and then using theirs - I'm a baaaaad pixel pusher; too impatient) I'm certainly up for one of the early chapters. Possibly about a monk of some description...
