Nah, Sonic wouldn't look in place in a SSB game. The only sure-fire characters I think are going to be in the next one are Wario, Waluigi and Diddy Kong.
What really fascinates me is the online prospect. If Nintendo set up leagues for seperate games, it would attract a hell of a lot more people, and considering that SSB is one of the most popular games ever, it would certainly work in Nintendo's favour.
And we can can all see that I'm the best at SSB on C3