I think there was a Mortal Kombat Kart a while ago...I dont remember...
oh yeah, I can picture it now... EPIC WIN! XDD
though I prefer the Nintendo Kart idea over the Brawl Kart idea as SmashChu stated for 2 reasons:
1. it wouldn't give Brawl a bad reputation
2. Snake and Sonic were added for different reasons (Kojima begged for Snake, Sonic fans begged for Sonic...
)
there could be a single or double kart option (for those who liked/hated the MKDD idea)
as long as they don't go crazy with spin offs... (next is Nintendo Tennis, Nintendo Golf, Nintendo Baseball, Nintendo Soccer, Nintendo Basketball, and finnaly the oh so dreaded Nintendo Party! oh noes!!)
on an unrelated note I wasn't exited when MK Wii was announced since ROB was removed...lol