Since we're talking about potential characters now, I'll chip in with my 2 cents.
The obvious first 2 are Mewtwo and Ridley. They are fan requested and are good representations of modern franchises.
Thinking of other modern franchises, other thoughts come up. The other oft mentioned characters are Chorus men, dixie kong, and imps. Chorus men are a representation of Rhythm heaven, which is not only a modern franchise which currently has no character representation, but already has an enemy in smash run. I say its chances are high.
Dixie kong and impa are characters in modern franchises as well, and they definitely have chances, but I'm not sure on how high those chances are. Impa, in particular, I think won't happen. unless she's based off Hyrule Warriors, but that is a bit too recent and they the chances of her being a part of development from early on is a bit slim. She could be SS imps, but that might not have very much to it that's different from sheik, and Sakurai said he wants to include unique characters as a focus. The exception is, of course, the 3 clones, which are literally glorified palette swaps. I'm not familiar enough with Dixie to have a real valid opinion on her, unfortunately.
On my personal wish list is Demise from skyward sword. We haven't had a new zelda rep in this game, and I think Demise makes too much sense. Sakurai most likely knows how much certain people want a ganondorf with a sword, and we all know that in this he's still falcondorf. So what if Sakurai did this planning to make Demise (who's essentially ganondorf in the game) the ganondorf with a sword we always wanted? Not sure on his chances, but I think he makes a lot more sense than Impa, and I REALLY want him in this game.
I really think, above all else, heavily requested characters are going to be saved for WiiU. That way people who wanted them specifically and have already bought the 3ds version are going to be compelled to buy the WiiU version. I think Mewtwo, Ridley, and maybe Demise fit this, and I also think Chorus men are one of the most likely. Since, with Mii characters added and all 3ds fighters unlocked, there's room for exactly 4 more characters, I guess you could say Mewtwo, Demise, Ridley, and Chorus men are my wish list.
and He states he is the Tower of Smash Leaker and he called Smash World so he is Decently reliable