Here are a couple of stages that I think would be really cool:
Pinball:
Representing the Retro Series, I think with the Wii U's graphics, this stage would look absolutely amazing! Well, that is if they were to "revamp" the colors and scenery. Anyway, the pinball would be the stage hazard as well as the flippers.
Battle Arcade:
Representing the Pokemon Series, this stage isn't very likely but could be a fun stage. Just like in the game, the screen will flash with different weather elements and affects. Which ever affect the cursor stops on would determine what happens for some time. Heck, Sakurai could change some of the affects to match more of Smash's status conditions. Like I said, very unlikely and not really deserving, but I believe this would be neat.
Mine Cart Carnage:
Representing the Donkey Kong Series, This is one of my most wanted stages. The stage would set on the tracks and different enemies from the DKC would zoom by in a cart. Collision would cause extreme knock-back.
These are just a few stages that I feel would be cool. I'm not saying these are the most deserving stages(Like Bowser's castle, I mean why hasn't it been in yet?!).