My idea:
You start off in a plain area with no platforms. as the characters make their appereance, you hear an announcer saying: "3... 2... 1... GO!" In that second the stage will start side scrolling somewhat fast (just enough to make slow characters like Bowser and Ike to make it too) and the characters will be running for waht looks like a minute or so, the track could even have boosts and loops :D. After running through the whole track you reach a somewhat basic battlefield in which you may fight normally.
I think this is a pretty good idea, some people might even use the racing part as a race "mini-game". A little like the Ice Climbers stage in Melee except, it's sideways and finally stops moving after some time.