If it were up to me, it would go back to only having Nintendo characters, as that was the original intention. Same with Mario Kart. I think it was way cooler when Mario Kart was this little fantasy where all the good and bad people in the Mushroom Kingdom forget their quarrels for a little while and take up racing. But when they started adding Link in there and a Mercedes? No. Just no. I mean, the game is great still. Everything about it is legit. But when you start branching out into the fourth-wall, or adding third party characters, the immersion is lost for me.
On a related note, Mortal Kombat is guilty of this as well. I mean, Freddy in the Outworld, trying to kill Sub-Zero? Come on.