Ever thought of putting coins for example, players join to buy cars in the game? or skins? I saw that it has 80 models .. some goals would be interesting, more not now, maybe more ahead .. :D
I am already working on smoke effects as reward for doing certain things. But cars should always be random. It's just different amount based on your position on the end of the race.
I do not like idea of currency in game since I see it in every game and it's too boring. I feel like current approach is better.