There's already Formula K, which is rover circuit racing. We use PersistentTrails instead of multiplayer for ease of entry, due to there being no time-zone limits to turn-based races. If that's interesting, check the thread in my signature out. If not, I do see the fun in wheel-to-wheel rover racing, however I don't think there'd be big fields. There are many time-zone problems that arise with playing together in Minecraft which would carry over to this. Also: plugins/mods for cars are Tweakable Wheels, Wheel Telemetry and Rover Wheel Sounds. Anyway, awesome idea, and if the earth were flat I'd absolutely love it.