Please stop me if someone else has already mentioned this, but I think it would be fun to be able to buy yourself a ride in the game, like skateboards, cars or horses... And maybe make it so every different kind of vehicle had a different special characteristic, like cars allowing you to move around faster, horses allowing you to jump higher, or skateboards allowing you to... uh, look cooler? :P
This idea could even double as a racing mini-game, come to think of it. Probably racing against other players in real time could be too hard to pull off because of synchronization/lag issues, but racing against a computer AI or against "ghost" racers (saved replays of previous runs performed by your friends) could be interesting nonetheless. :3