I don't like that they made Luigi the cowardly one. They should give him a game where he is actually heroic. When did he start to be portrayed as a coward anyway? He used to be exactly like Mario just taller and green.
It's a funny thing to think about but there is no point in making Luigi Kart because it would be JUST like Mario Kart. That'd be like making Luigi Tennis or Luigi Golf. It would utilize all the same characters and gameplay just with a different title.