Ok, first off I know a lot of you won't believe me here as clearly you have no reason to. Let me just say that I have it on good faith that the information I have received from my source is legitimate. I don't know everything there is to know, but I know a fair amount.
There's some pretty juicy stuff here.
Mewtwo is back.
No one from Brawl has been cut yet, and there are no plans to cut anyone. Although some characters are considered "lower priority" so if time becomes an issue we may see one or two cuts.
Leading on from that, Pokemon Trainer is the same as he was in Brawl.
Pac Man is playable. Ms Pac Man is an Assist Trophy.
Palutena is also playable.
Ridley is also playable. His size in the game was described as "appropriate". Whatever that means.
Dixie Kong is playable, and plays NOTHING like Diddy.
Mew is a pokeball.
Isaac will return, but again as an Assist Trophy.
Mii's are in, there is a degree of customization to them although
Takamaru will serve as the "retro" themed character for this game.
There are also several characters that are still being considered to add "late in development", one was Toad, another was another DK character (I wasn't told who) although that clearly means that both could make it in, or neither.
Several characters will be getting alternate costumes, this includes both Dr Mario and Dr Luigi.
If I receive any more information then I'll let you all know. I must stress that these aren't the only new characters, but that was all I could get out of my source for now.