Pretty much a very small niche MMO.  Past racing games have fallen flat on their face, although there is always hope a team can put together a game that involves more than just racing on a race course.

to be honest i don't see how any of these silly niche games could be MMO's.  I think big multiplayer games with ladders and leaderboard are the way to go, not paid subscriptions for racing cars (seriously, even people who play those games probly don't play more than a few hours a week).