sounds like a great idea - probably a lot harder to do than one thinks - but could be fun ... and if it works out well, it could be used more often as a challenge. And I propose (just to get it out there) that people only can join one team
And even without the challenge part it could be fun to make a collaborate map (on a smaller scale than the world building that is allready taking place) where for instance you have to expand the current map in one compass direction (or just go east for everyone).