There are so many ideas in this thread I don't know where to start....
I'm all for keeping it simple.....I think have an open and street class with both safety (cage, seat, harness) and tyres free, and under each have a fwd, rwd, awd class. The determining factors that put you in open or street should be exterior panels (minus front bar, rear bar and wing) and stripped interiors. To go further than that you could start looking at things like fuel systems, brakes etc which sort the higher hp cars from the rest.