So here's what I'm think on stages.
The confirmed new stages:
-Battlefield
-Final Destination
-Mario Galaxy
-Mushroom Kingdom U
-Skyloft
-Pyrosphere
-Great Cave Offensive
-Orbital Gate
-Kalos Pokemon League
-Coliseum
-Gamer
-Palutena's Temple
-Garden of Hope
-Town & City
-Boxing Ring
-Wii Fit Studio
-Gaur Plain
-Pilotwings
-Windy Hill
-Wily Castle
-Pac Land
That's 21 so far. The 3DS has 25 total, if you were wondering. The rest of the new stages, I'm thinking are split into a few categories.
Past character franchises without a stage in either version yet (barring R.O.B. for obvious reasons):
-Donkey Kong
-Yoshi
-Game & Watch
Hinted at via the list of character/stage franchises in the 3DS version's code that are unaccounted for:
-Wrecking Crew
-Miiverse
-Wii Sports Resort
Likely further franchise representation for the 3 most popular franchises:
-Mario
-Another Mario
-The Legend of Zelda
-Pokemon
That would leave only F-Zero and Earthbound as the franchises represented solely through returning stages (and R.O.B. and Duck Hunt would have no stage at all, but whatever). This is what I think is a relatively likely outcome. It would give us 31 new stages, equivalent to Brawl.
But there's some leeway back and forth on this. I'm not entirely convinced that there will be another Pokemon or Zelda stage--it may be that in order to represent more franchises they had to dig into the more well represented to find time and resources. Mario may get along with only three as well.
Donkey Kong and Yoshi I'm 100% on--the original trailer for SSB4 had a new rendition of a DKCR-esque jungle, so they have to be thinking of environments there I would think. Plus, I mean, come on, it's Donkey Kong. The only way Donkey Kong has no new stage in either version is if Sakurai has completely lost the plot, and I don't think he's there yet.
On Yoshi we know there is a new rendition of the Yoshi's Island level theme that was heard in the direct but is not at all present in the 3DS version, so it has to go somewhere, and SSB3DS took the one past stage it could really fit on (and we know the two versions don't share past stages). Plus, the only other past stage Yoshi has available to pull from Brawl is Melee's Yoshi's Island, which is a Super Mario World stage and is more a Mario stage than a Yoshi stage, really. I don't see that being Yoshi's only stage.
With the hinted at franchises, I'm least certain on Wii Sports Resort, as it is kind of redundant with Pilotwings and it may be a dropped stage concept that was merged into Pilotwings. On the other hand, reusing Wuhu Island would be a cheap way to make more stages and is kind of in character for Wuhu Island if you think about it, so that's certainly a possibility.
I was hoping that Magicant and Mute City would make it into the Wii U version, but I'm afraid that's probably unlikely. I can't help but feel that if Magicant were in the Wii U version it would have been shown on Ness's character page on the official website, but it's not, just Onett. I think Onett will be the only Earthbound stage.
That's my little thing on stages, I need to go to bed, gotta get up early tomorrow.