I only heard this from a friend who has a few connections (I can't say who, it'd put their position in jeopardy, just trust me they're reliable) so I can't say this is true for sure. Take this with a huge grain of salt. I know you probably won't believe this anyway, but this could be true so I feel like I should share it anyway.
First of all he told me that Palutena is in the game. Sakurai originally wasn't planning on her but he was pretty flattered after seeing the massive fan support for one of his own characters, so he decided to add her.
He also told me that Ridley is playable. He's the largest character in the game. Basically the polar opposite of Little Mac. He is slow on the ground but gains speed in the air, his aerials are seriously powerful, I was told he's definitely at least going to be B tier. Sakurai started considering him after the interview where he was asked about Ridley, and decided that he was unique enough and Metroid deserved it.
Chrom is the Fire Emblem newcomer, and he will in fact be revealed in the April Nintendo Direct. He actually plays a lot differently from Marth and Ike from what I was told. He uses a system where he can switch between his sword and lance, it seems very interesting.
Takamaru is the retro rep, added because of his popularity. His design looks like a mix of his original NES appearance and his appearance in Dynasty Warriors, with some of Sakurai's own original things thrown into it. He looks really great. I couldn't get any information on his moveset, they're likely still working on adding him to the game.
Pac-Man and Miis are also both in the game, and both will most likely be revealed at E3. He hinted at a Pokemon newcomer too, but I don't know who it is.
Shulk was considered at one point but Sakurai didn't think it would be a good idea since he's only been in one game.
Aside from newcomers I have confirmation that Ice Climbers, Ness, Yoshi, Wario, Meta Knight, Pokemon Trainer, Sheik, and Zero Suit Samus are all starting characters and will be revealed sometime before release. Ganondorf, Ike, Captain Falcon, Falco, Wolf, Mr. Game & Watch and even Mewtwo are all fully programmed into the game right now, but they are unlockable characters and won't be revealed until after the game comes out. Jigglypuff and R.O.B. are planned to be added at a later time. Ganondorf is decloned but I'm not sure how he was changed. They're trying very hard to bring back all of the characters even better than they were before.
Mewtwo and Lucario both have their Mega evolutions as their new Final Smash.
Realgam Tower, Bowser's Castle, Nintendo Land, and a stage based on Pac-Man World are all stages for the Wii U version.
Pac-Man's 3DS stage is simply the maze from the original arcade game.
In total, there are about 50 characters, excluding transformations, so there are a few newcomers I was not told about.