This is one that has A LOT of possibilities.
A few things I can think of:
If you can, at least right away, try to get a demo out that you can "restart" with a certain key. I want to play it over and over, but, restarting it each time is a chore. Plus, one time it froze my system the second time I played (Win XP).
I can see definate multiplayer in this (as I'm sure you have), and hopefully network play soon. Also more cars and different "platforms", like maybe an "arcade" mode where you can get to different "levels" with cool bonus stuff as you get more cars off each level.
I can't wait to see this progress into a full game.