Now I know what youre thinking. Im a fake like all the others. Well, Im not. Im an actual developer of Namco-Bandai working on SSB4. I know the entire roster Sakurai has planned. Here are the characters planned:
Mario/Yoshi/Wario series: Nothing new except Rosalina.
DK series: Dixie and Kiddy are a newcomer tag-team, who act like a combo of the Ice Climbers and Zelda/Sheik. They are a transformation character, but the character not being used attacks as a CPU.
Zelda series: There will be a newcomer, Midna. She rides on Wolf Link, and is very unique because of the mount. Actully, she was the only reason Sakurai chose the Twilight Princess designs over the SS ones!
Metroid, Kid Icarus, Kirby, F-Zero and Pikmin series: Nothing new here.
Star Fox series: Fox and Falco. Wolf was cut.
Pokémon series: Pikachu, Jigglypuff, Lucario, Mewtwo, Blaziken. Pokémon Trainer was cut. Blaziken borrows moves from Infernape and Emboar, like Heat Crash.
Fire Emblem series: Marth, Roy and Tharja. Ike was cut. Roy was chosen as a fan-favorite, but sadly, he has a moveset ported from Melee, meaning he will be a clone. Tharja will be very unique, and was chosen to be different from Marth and Roy.
Earthbound series: Just Ness.
Retro series: 2 newcomers: Muddy Mole and Duck Hunt Dog. Muddy Moles digging ability is implemented into his move set and, I must say, is very fun to play around with. Duck Hunt Dog is also very interesting in that you can target people with the joystick and shoot them from the 4th wall.
Other series: The Little Mac/Pac-Man/Mii rumor is true. Miis have a customizable moveset, although is is balanced out very well in my opinion. Little Mac has a very unique gimmick of Star Collection as well, where he can get stars by dealing damage. Those stars improve his Neutral and Up Special's power. Snake's been cut as well.
Assist Trophies: All the assist trophies from Brawl return, except Gray Fox. In addition, there is a new Kid Icarus assist trophie, the Eggplant Wizard. Many other trophies are added as well.
There are also bosses only on the Wii U version that a 5th player can control with the Gamepad. The bosses include Hydreigon, Yin-Yarn, King Boo, Dr. Kawashima, and of course Yellow Devil. In the adventure mode on both versions, before fighting Master Hand, you fight the new Fighting Android Crew, which are 4 robots that can transform into random fighters at will. On the 3DS, instead of the boss battles, Target Smash will return and will be like Melee's, but with only 1 level per franchise.
So, that's about all I know, but I might know any answers to questions you ask in the comments! Feel free to ask anything you want, but I still might not know it. Peace out!