I have been considering a challenge idea. This is a bit larger than some of the others (from what I have read) and I am not sure if there are enough people to even get this off of the ground. Please note that I have not thought this through in its entirety, I am merely making suggestions which we can take and run with...
World-Building to Completion
This is actually a series of challenges, taken one at a time, that will ultimately combine into a fully fleshed out world/campaign setting. It starts out with a challenge to conceptualize the beginning of a world in its most basic form; geography. This may include illustrations/maps/etc. but does not include specifics like names or maybe not even geographical landforms such as rivers or forests. At the end of the challenge, the participants vote on which one they believe has the most POTENTIAL.
That then becomes the basis for the next challenge...creating a fully fleshed out map/geography. Again, the participants vote on the product with the most POTENTIAL.
Next challenge may be political or racial boundaries, pantheon, history, etc. The process is exactly the same for each challenge; participants vote on the product which offers the most POTENTIAL.
In the end, everyone has contributed to something grandiose created through friendly competition and collaboration.