It would be interesting to do remote control. I.e. nobody in the car. Two classes: one with restrictions (i.e. driver must be in control - no assists / etc. May be tricky to define.), one without (want to go fully computer-controlled? Sure!).
Although it would be interesting to go "you must have a space where a person can fit, and if you ever kill the dummy inside you lose the race" (defined a little more precisely, of course).
Remember: you probably want to keep the scale vaguely car-size. That way you'll get the most applicable innovation coming out of it. And also that way it'll tend to be the most interesting to spectators.
