Worth a double post...
Perhaps a community vs pro's mapping event? Valve Team vs TF2M team?
2f2f doesn't even exist anymore
Collaborations don't work well, at least in the community - not sure about how Valve do it.
Why not just have a 72hr mapping competition, with one or two Valve mappers, and open for anyone to join? Then we could do what grazr suggested (and I really like that idea btw) and have the comp community play the map (or better yet - a 6v9 thing like what happened a while ago) and some commentators commentate.
Which map? If we get a 72hr contest set up, and valve mappers enter, there could potentially 100+ entrants. The voting would be ridiculous.
But I'm game.