Unless Sega falls out with Nintendo (which I doubt will happen anytime soon), I'm sure Sonic will stay as a series veteran.
And with Capcom at the risk of closing down, and people wanting them to do something with Mega Man, and them not delivering, I'm hoping that Nintendo buys the IP or something (that'd help Capcom with their financial debt if they sold Mega Man to Nintendo). Because I think they could revitalise the series, and he looks right at home among Nintendo's cast of characters (as well as with Sonic). Heck, because of how true Mega Man's movements in Smash is to his own games, and because of how AWESOME his model looks, I honestly think an excellent Mega Man game could be made within the Smash Bros engine (which was why I hoped there would be an Adventure Mode in this game...we're not getting a story, anyway, but we do know the 3DS version has a platformer mode of sorts).
Sonic was my main in Brawl. I honestly thought he was underrated in terms of his fighting skills (I beat Boss Battles on Intense with Sonic). Yeah, his moveset could have more than variations of the spin attack, but all of his attacks are useful in some way. He is really one of the characters meant for veteran players, he isn't easy to use. With practice, though, he's a beast. And the overall faster pace of the new game should really give Sonic even more of an edge. I've already noticed improvements, and this isn't even the finished version yet (in his Miiverse post that revealed Princess Peach, Sakurai confirmed that the movesets shown off so far are not final).