Upgrade
Kind of boreing, no sound at all. Like give the cars diffenent engine roars, and give screeches from tires when coming around a turn. But most importantly, offer upgrades. And not just little upgrades, go all out, start out with a four cylinder then work your way up to a twelve. Have add-ons such as turbochargers and superchargers. Customizable Nitrous boosts. You can upgrade for steering with better tires. And if you really want to go crazy, add weapons like oil slicks and missles. Blades that come from the side of the car. flamethrowers, machine guns, anything.