I like the race track idea. It's easy, simple, it's in style and it's something we all can work on that can be done relatively soon. Everyone takes their own section and just builds on.
My plan;
Someone starts, I'll just pick me so we can have a base, I will build the starting spot and the like, "race center", make it look all nice. The next person, say, is Legodude77. He'll build his section of the track, with his own theme and his own obsticles, and so forth.
Make it possible to race but not impossible, give it your own style. That's something we haven't done, do your own style.
I'll be out on a date tomorrow afternoon but if we can get it started in the morning that would be great.