I'm a Sonic fan. I really hate the idea of a second character rep. Item - yes, Assist Trophy - Yes.
Other character? No.
Even if Nintendo bought it, Sonic is sonic. He defines his series, and the other characters are merely similar entities (minus Eggman). Tails merely has a Tail, that's as unique as he gets. Knuckles has ... knuckles. Shadow is just an evil sonic, his negative, sadly (SA2 is my favorite game though ^ ^)
It's like this, would they add another Pikmin rep? What would the others have that Oli doesn't? Although there's no formula, it kinda helps make my point.
Sakurai is strict on third party characters anyways. Giving a franchise two when most Nintendo ones (Samus, Falcon, Olimar) is unwarranted. It's Nintendo All-Stars. Sonic and friends are not All-Stars. Sega can make a Sonic fighting game for that in my opinon and have Mario around as a Nintendo Rep. And the Olympic Games series satisfies the Sonic characters, even down to Vector lol.
I don't see a plausible reason for another Sonic character either! What would they bring to the table? Sonic would not sell better because of it, and giving us Tails when there are more excitable choices such as Pacman seems illogical. I'd love to have a Sonic fighting game, but Sonics all thats needed in smash.