Sonic was put in to serve fans, yes. But as a early Sonic fan (Down with 3-D Sonic games! Except Sonic Adventure 1 and 2) and BIG fan of his DS/GBA entries (Sonic Rush and the sequel, and Sonic Advance 1-3 were almost as good as Sonic 2) I was still surprised he made it in.
Though his specials are questionable. I was expecting Up B to be the Light Speed Dash but the Spring is serviceable, I guess. I still don't like how that gave him a Spin Ball AND a Spin Dash. IMO, Spin Ball is better and forward B should have been Sonic Wind or something.
But what's done is done.
Tails and Knuckles, I'd willingly accept and look forward to.
Shadow and Silver, I rather be color edits/costumes for Sonic. I'd be hard pressed to accept Silver as a PC in smash. Not so much Shadow, but I wouldn't ask for him to be playable either.
Would honestly want to pay as Blaze more then Silver or Shadow. She has better potential to be "unique" rather then "Sonic Cloned." Course, C. Falcon and Ganondorf are from two different series. >.>
Robotnik would make a nice boss or stage hazard. Not so sure about a PC.