Let Mario Kart be Mario Kart, please. I don't mind some cameo appearances like R.O.B, Link or the Inklings, but keep the company wide crossover stuff to Smash Bros., thank you.
There's still plenty of unique Mario universe characters to choose from, it's just Nintendo likes to be dumb for the sake of being dumb and put in stupid shit like Pink Gold Peach. There's still characters like Diddy, Dixie, Cranky (if Funky could be in MKWii, why not Cranky? lol), Nabbit, Pauline, Professor E. Gadd, and such.