Why not include cars you can win? That eliminates alot of cars, most of them being all the good cars, and for rally that leaves out all the good rally cars. And you said, "any car, it's all about the skill". If we included the important specs of the car, namely weight and horsepower it won't just be best time as the indicator of how good someone drove.
I like the idea 'standard' course, a rally course, and a speedway course. Maybe two standard courses though, one that has lots of tight turns and one that doesn't. Or people could just post their times and info for their favorite courses, and then others can add more or challenge those times. Like pick and choose, and it'd be fun to see times for lots of courses. But I guess have the 3 or 4 courses being the main challenge ones. Setup something like this:
Main Challenge Courses:
- Super Spdway
- Apricot Hill
- Laguna Seca
- (rally course)
Drivers Favorites Courses:
- (etc)
EDIT: We could also have the main challenge courses vary over time, like a new one added each month, or a drivers favorite course that has lots of entries be promoted up to a main challenge one.