There has been a leak going around about new characters and stages on the Wii U that you can transfer to your 3DS when both games come out. The leak says there will be a new mode, "tower of smash", which you can read about in the leak. It also says Ridley, Impa, Mewtwo, Dixie Kong, and Chorus Men will be playable. It talks about what their moves will be and everything and also has a list of about 40 or so new stages. You can read the leak here[1]. I will be posting updates on proof confirming or deconfirming the leak.
Game and Wario[]
The leak says there would be a stage based off of Game and Wario. Today we got "Gamer" which is from Game and Wario. It's only the first possible evidence to this being true, so it doesn't mean anything yet.
Onett and Smashville[]
The leak said Onett and Smashville would return, and sure enough they're confirmed.