for the multiplayer races, why not just go back to the old days of 2 player sonic the hedgehog style races, where instead of having them side by side, have the track over and under?? so its the same identical track but one of you races on top, and one of you races on the bottom.
also i think that, for the extra axis control, (up and down) you should have an option of different controls, like normal mode for how its played now, or expert mode for using up and down aswell, because i could see a lot more people complaining about how hard the game is, as they do now, it would be even worse.

