I've been itching to do something with black and white/white on black. Maybe black and white star blocks? One block would be black on white, the other white on black. Each person could sign up for a certain numbers of pairs, maybe?
That's just one thought. I'm open to ideas, but I wouldn't participate in an "anything goes" color scheme. I'm afraid I'd end up with blocks that clash (ie if some were patriotic and some fall, and some floral, you get the idea).