Going to organise this so it is understandable:)
New types of floor: Glass - very fragile, full speed or hard impact can break it, slippery but gives off more speed
Asphalt (road paving, whatever you call it) - gives off increased grip and significantly faster
Snow - winter edition of mud, should be easier to go through, very little grip and speed but a bit more "bouncy" (less downforce)
Rock - harder to drive on, unstable and uneven, less speed but more grip
New "weather" systems: Sunny, Rain (less grip), Wind (slows player), Cloudy, Snowing...
New "cars": Maybe add a bicycle, older car, truck...
Larger domes: Sometimes we just dont have enough space lol
Larger selection of gadgets: Maybe add electricity, wires, brake pads (if the player does not have enough time to brake, brake pads slow him down under a second)
Larger selection of decorations: Candles, Hot rocks (not lava), Stars, Paintings, larger selection of signs, More types of rocks, trees, Vines, Big trees, Sculptures....
Time cycles: Day, Dawn, Night...
New types of background: Swamp, City, Rural, Tundra, Mountanous...
Be able to customize the color of your "car"
Be able to customize the color of skin of your alien
Be able to customize more than hats (shirt, pants, shoes...)
Be able to customize the look of your car (stickers maybe
Be able to race against your friend in real time
Be able to race against your team in real time
I have many more ideas, but this is getting very long already so i will stop here :P