Sonic is my favourite franchise alongside Smash Bros, but I don't see a need for another Sonic rep. I wouldn't be against it if it happened, but I seriously doubt it will, and I'm not asking for any. Sonic in Smash Bros is really cool both because of history reasons and because I get to see Sonic interact with some of my other favourite video game characters in one of my favourite genres of games. I've already seen Sonic fight against Eggman countless times, or fight alongside Tails and Knuckles and Shadow tons of times, so Smash Bros would just be another one. And Mario & Sonic already have their own crossover series anyway. And besides Mario characters, there are no Nintendo charcaters I have any real interest in seeing Sonic characters interact with, except for Sonic himself (and maybe Eggman).
A second assist trophy would be cool, and if there is a boss battle mode, a fight against Eggman would be amazing. And a handheld Sonic stage for Smash 3DS would also be cool. Beyond that though, Sonic doesn't need any more representation, and in the Trophy/Sticker/Music department, he had a lot in Brawl, and likely will again. So Sonic franchise representation is fine.
If they had to put a second Sonic character in, it has to be Eggman. He's the only one who could be a legitimately unique fighter in Smash, on the level the 5 confirmed newcomers seem to be, he'd be an awesome villain to add to the roster, and any excuse to hear Mike Pollock's voice is a good one! Though I don't want another Sonic character because I am hoping they will confirm one more third party rep (besides Snake, who I believe will come back anyway), and I'd rather it be a different franchise. My personal hope is Professor Layton, as he is so strongly associated with Nintendo (much like Mega Man), he is an awesome character with a layered personality and a lot of back story, and he has a ton of potential for a unique moveset, and it would help draw in more 3DS owners. It'd also create a balance in the third party representation, with Sonic and Mega Man being the retro third parties, and Snake and Professor Layton being the more modern ones (even though Snake is technically retro, everyone remembers him for Metal Gear Solid onward).
I'm going to be the devil's advocate and say Pac-Man does not really deserve a spot on the roster. As always, I would be happy for the people who want him if he did make it in, but I personally think he does not deserve a place because the first Pac-Man game is the only one anyone really cares about, and that game is not popular because of it's mascot, while Sonic and Mega Man are very much popular because of the appeal of the mascots. They would probably just use Pac-Man's new design too, and the new Pac-Man is honestly not that cool at all.