There have been a few attempts but they never seem to go far. I know in the past year or two there have been 2 auto racing themed GBs, and a 1950s GB. The endurance racing and 1950s builds started out ok, but kind of petered out, the Rally GB never really took off.
As you may have noticed the auto section is not the busiest of forums on the site, so I'd suggest keeping it fairly broad. I would probably participate just to show support for the auto forum but it would ultimately depend on what the actual GB was about.
I kind of like the idea of a commercial vehicle build. Semi's, delivery vehicles, tow trucks, emergency vehicles, etc it could probably even include many race cars (hey they have corporate sponsor decals, that is commercial
). Speed shop delivery vehicles could keep the hotrod guys busy, a junk yard parts truck could make the weathering guys happy, and if someone really wants to build a sports car, they can, it just has to be behind a tow truck.