Test Drive Unlimited with GT's physics model would be absolute car porn and fit exactly with what Kaz would want as a celebration of motoring. To be able to USE your car for the transport aspect, to get from A to B to various places, such as the car wash, gas station, events, auto clubs, garages... instantly the non-racing and non-sports cars would have a place in the game. This would be a HUGE plus to the world of GT.
Do it. But do it in GT6.