this is a great idea.
I think there should be two different leagues. one for modified cars and one for stock cars. I think the stock league would fairly pit drivers against each other to test actual driving skills. I dont want it to get too complicated for you, but each league should have different classes as well. so someone isn't entering a Honda civic against a formula gt or x1.
also, if you have moderators here in the forums, they should try to be present in races. maybe making some moderators as race officials/judges would be beneficial to keep track of things. they would attend races as spectators to monitor results and keep an eye out for shenanigans.
as bmx13chris pointed out, different racing types should be included. road, drifting, rally and carting.
I think as this forum continues to grow these racing leagues could become pretty big. should be fun.