I think Sonic, Pacman and Mega Man are just about guaranteed to return. Sonic especially considering he's been in two games now, wouldn't even be surprised if he got a second character.
Ryu, Bayonetta and Cloud are the only ones I'm really worried about. Seeing how it's becoming more and more likely that this is a new game entirely, they're probably gonna have to prioritize some characters over others, and I doubt those tree are high on the list (we would've seen them in the base release otherwise).
As for new third parties, I dunno man. There's not a lot of characters (if any) that could match the massive hype that characters like Mega Man, Cloud or Sonic. Aside from Bayonetta and maybe Cloud, all third parties we got are ****ing legendary and instantly recognizable even as pop culture icons outside of gaming. I know that a lot of people want to see stuff like Shovel Knight, or Shantae. But if you seriously think about it, they have nowhere near the history or mainstream appeal than WiiU/3DS's third parties.
We might get a surprise at the end of the day, but I don't think there's any single guest character that can top the ones we already got. (I'm trying to be objective by the way, I personally wouldn't give a **** if they cut Cloud or Sonic or Pacman and would be hyped as **** if we got Shovel Knight. But they choose these characters extremely carefully, we have to know what to expect).