Do you mean having an open area to drive around in outside of races? It seemed that a lot of the 5th generation kart racers did that (Diddy Kong Racing, Crash Team Racing, Lego Racers), so it could work. Probably not what Nintendo wants to do with Mario Kart though.
At the top of my wish list for Mario Kart 3DS is for the branching track design to be expanded upon. There were some small moves in that direction with Mario Kart Wii, like the track in the mall or the volcano, but I think they can go a lot further with it.