Well this certainly did answer my initial question if there was any interest in this sort of thing. XD
I appreciate all the enthusiasm, but for now let us focus on my initial idea. That is, basically a glorified subforum where a bunch of people workshop their way through the challenge. While all of those collaborative worldbuilding game ideas sound fun, this is not what I am planning here.
The idea for this setup came when I was thinking about doing the aforementioned challenge and was looking in to ways to organize everything. I thought about just creating a private game and use threads to keep things organized. Then I thought about inviting some others along for the ride.
That is really all I had in mind here.
I would set up the game, set it to private so that no spoilers leak, give every participant a subforum and set up a general chat thread. The rest would be up to the participants.
The challenge is ultimately just a framework to pace out the workload and extremely flexible as to what you end up doing with it. You can add things you really want, leave out or change parts you don't need. There would be no obligation to keep pace or anything of the sort. Though everyone working on the same part of the challenge at the same time seems kind of fun to me. How much input you want, anything from quick feedback to full collaboration, is once again up to the participants.