How about the possibility of hiring car tunners (such as Brabus, Mansory, Ac Schnitzer, Techart, Hamann, Edo competition, Novitec) to increase the PR of the cars.

For example, you already must have the car fully upgraded. Level at least 500.

Charge real money.

And each 50 levels you goes up, would allow to increase further. Like Stage 1, stage 2, stage 3, and beyond.