My ideas / questions:
1) How would stunt mode work? I don't know if it would fit in too well with the physics but idk.
2) Painting your cars with images could be a cool idea as long as you cant just use any image bc otherwise the images could be something inappropriate/disturbing.
3) What would you be able to buy with the in-game currency? Skins for your vehicle? Also how would you get the currency? Could be interesting..