Now bouncing back off of the previous thread made by Demonz just a bit, I do believe it would be very nice to have separate categories for each kart, due to each kart having a unique gimmick (i.e. the broom being able to go in water or bike being able to wheelie now).

I also believe a cup category should be implemented for the new Grand Prix mode that allows you to play all four tracks simultaneously. Time could go by the time taken to complete each map, starting at the beginning of the first map, and ending at the end of the last lap.