Why would you want that? A game is made with a certain idea and style behind it and that will probably work best for the game. Mario Galaxy with a killzone brownish look will probably benefit the game as much as killzone with a mario color pallet.
Well obviously because the gameplay appeals to me much more than the style, and that goes 10x for the main character. It's not like there are no games whatsoever out there that at least allow you to change the look of your main character, or even choose between a few different ones, especially in the platformer genre.
Anyway, I'm probably the only one who feels this way about Mario, so I'll shut up about it. I'm just giving you the biggest objection I have against Mario games. I can often see what is appealing about the gameplay, but often I really can't stand the aesthetics. I do like some of the graphics in Galaxies, but the enemies and the main character have almost always in almost all games really, really not clicked with me.
And so here you found me wishing that the gameplay of Mario would come in a new package / skin.