Audi TT with a couple of bodywork changes, and an entirely different drivetrain.
Bodywork: A splitter and floor. The latter has venturi channels and leads to an aggressive diffusor. This of course requires cooling exits in the bonnet. Inbetween the venturis the there is a skirted section in which fans suck the air out and thus create downforce. But the fans are driven by computers that take the current suspension loads and acceleration the car experiences as input to increase or decrease downforce as needed. Other than the cooling exits, the diffusor and a little spoiler at the rear to remove unfavorable pressure gradients, the car shouldn't have overt aerodynamic devices visible. Oh and it's a convertible, of course.
Drivetrain: I have two ideas. No.1 is a cool modernized ICE and nr.2 is electric.
1) V8 4000cc TSI engine fitted with a rotary valves, integrated exhaust cooling etc. - that should easily produce 600bhp. The engine drives an infinitely variable transmission (or whatever autogyro has come up with, which reminds me that I wanted to read that thread...) with a torque sensing diff and all-wheel drive. A computer constantly checks whether there is enough traction on the outside wheels and if there is, it directs more torque to them to make getting around corners even more easy.
2) A very flat flywheel mounted low on the floor (for CoG) and motor/generator units in each wheel. Once again, torque vectoring will be done via computer. Power is transfered to the car trough magnetic induction coils along the floor. The flywheel should be big enough to go at least one lap in the Green Hell and/or on the Curcuit de La Sarthe at full speed. Recharging is either done in the pits or on the start/finish straight.
