How would you do a GT mmo? It's a racing game, it would be multiplayer with leaderboards or whatever. Completely limited by the nature of the game. Unless avatards can all get out and wander around the pits and work on their bbq skills or something. That would be awesome. For reals.
Motor City Online done right I suppose, and maybe mix it with a Burnout Paradise-style open world as a hub, connecting to various race instances. One of the few good things I've heard people say about MCO is that it had a good economical system, which is something you don't really get out of just having leaderboards.
The other thing that's different when you do an online only GT, that could be postive or negative depending on personal preferences, is that unless you don't have an offline single player mode where you can just grind against the AI controlled cars for a few weeks until you have all the money/cars you need (and you also can't just reset the game anytime you total your car or something). I imagine that in a racing MMO, it would be a bit more challenging since you aren't going to be winning first in every race once you get the gameplay down.