It would be friggin AWESOME! Though I find it pointless in all these Mario platformer games that Mario always has to go through 80+ levels just to get to Bowser... I mean, why can't he just use a Raccoon Leaf/Cape Feather/Tanooki Suit/Wing Cap to simply fly straight to the castle?
I think it would be better to have just one giant castle level [like in Super Mario 64] and have it built to represent the different climates and environments [ice, water, cave, grass, sky, etc.].
Example, one area would be a grassy level, then you open a door and enter an ice level, then you open a door and enter a flying level, etc. Kind of like in Zelda: Ocarina of Time...