A new Luigi's Mansion stage based off of one of the mansions from Dark Moon. It could switch between the five mansions from the game, each with their own gimmicks or stage hazards. Alternatively, it could be based solely off of the last mansion, where it switches between the four rooms based off of the other mansions, as well as other rooms, like the ship, or the planetarium, or the toybox. (fyi my memory of this stage is pretty shaky) There would be ghosts floating around as hazards, each with their own properties, with their chances of occurring reflecting their frequency in the game (Greenies are the most common, and those ones with the giant brains or w/e are really rare) The original Luigi's Mansion will return as a Brawl Stage.
I was actually going to suggest that the Dark Moon stage be on the Wii U version and a Dream Team stage on the 3DS version until I remembered that Dark Moon is also on the 3DS. Not that this is necessarily a problem, but it seems kind of unbalanced having two Luigi stages on the 3DS version and no Year of Luigi representation on the Wii U version.
A Toad Town stage based on Paper Mario Sticker Star would also be cool. I'd prefer one from the previous games admittedly, but Sticker Star is the most likely choice, unless they do a Sticker Star stage for the 3DS version and a stage from one of the other games on the console version, which isn't really very likely unless they decide to represent Paper Mario as its own franchise (Which they probably won't)
A Bowser's Castle stage would be amazing. It would be really hectic, with lava and bullet bills and the spinning fire chains and so on. It could either be based off of one of the Bowser/Bowser Junior stages from the Galaxy games, or as a representation of the 3D Land castles (In which case it'll probably be in the already revealed 3D Land/NSMB2 stage)
Some new Sonic stages would be cool, although we're most likely just getting Green Hill Zone again. Maybe one to represent each Era? Green Hill Zone for the Classic Era, based off of the Generations stage this time. It would have more gimmicks based off of Sonic the Hedgehog this time, to be a better representation. Maybe on top of the checkpoint, there's a signpost that you can knock about into the air and attack opponents with? And stuff like crumbling platforms and the spinning spiked logs and stuff from the actual Green Hill Zone? Eggman could act as a stage hazard, coming out in his wrecking ball machine. You have to attack him until it blows up, at which point he flies away. Or he could have a multitude of classic Sonic bosses, which are completely randomised. Hell, I think GHZ could work pretty well as a scrolling stage, as long as it isn't excessively annoying like Mushroomy Kingdom (Although to my knowledge scrolling stages tend to be looked down upon so eh)
The Dreamcast era would probably be City Escape, due to SA2 being a fan favourite among the 3D games and the GUN Truck having a lot of potential as a stage hazard. SEGA seems to really be stressing Seaside Hill as the modern Green Hill Zone, so it's also a possibility, although it'll probably only be in if it replaces Green Hill Zone, which I don't see happening. Speed Highway could make a fun stage, but probably isn't happening, unless they decide to choose SA1 as Sonic's debut to 3D, in which case Speed Highway is the obvious choice.
The Modern Era would probably either be Rooftop Run or Planet Wisp, or possibly a Lost World stage. Crisis City would be an interesting stage, but considering what game it came from, it's probably not all that likely. Rooftop Run would make a fun stage. There could be barrels rolling down from the background. You could blimb up the clock tower (I'm sorry for all the scrolling stages, but I guess it's just what makes the most sense for a Sonic stage) while Eggman's robots fire at you with lazers. I'm not sure how Planet Wisp would be, though.
Each era would have music from their own respective era as well, probably from whatever stage is in Generations. And hopefully ice Cap Zone because Ice Cap Zone rocks. And maybe the Sonic 1 Credits medley. And obviously some Lost World music, like Desert Ruins 2 (Which is a ****ing amazing tune oh god)
An Epic Yarn stage would be fun. There would be parts of the stage that you can unravel, revealing platforms or doors into other layers of the map, where stuff outside it doesn't affect you. A Return to Dreamland stage is likely, but I'm having trouble actually coming up with any ideas for it. Maybe the volcano world or something.
I think I had some other ideas but I kind of forgot them :<