I think this would be a pretty cool idea. Let me explain.
Basically, Once a month there will be a limited car you could get, but you need to complete goals to retrieve those cars. Such as, survive 10 facility explosions or get 1st place 5 times in the Derby Arena. These cars won’t be ordinary. Each will be unique. Here are some examples that I thought of:
Skyline R34 Police Car
Koenigsegg CC Prototype
Ferrari 250 GTO SWB Breadvan
If you miss out on the event, you could still get the cars, but for Robux. I would say 100 to 150 Robux would be fair. I think 1 month should be a good amount of time for devs to properly 3d model and program the car.