I don't know... So far it's been only good news.
-Balance between Melee and Brawl
-Mega Man confirmed
-No tripping
Yeah, limitations suck, but I'm sure Sakurai will be smart and not fill the games' space up with movies when he could be developing more stages and characters as well as new mechanics.
Not all characters will return? Oh well, I'm sure he'll cut characters not easily missed. From Melee to Brawl he cut mostly clones (and an unfortunate Mewtwo who didn't make it due to time issues).
From Brawl to SSB4, I expect the same thing to happen, /''
So this is more or less who I see are in most danger of leaving:
RED status: (the clones)
-Toon Link (For being another version of Link)
-Wolf (For being more or less a 3rd Fox)
-Lucas (For being sort of like Ness, and also to keep things more international)
ORANGE status:
-Snake (Perhaps his reception wasn't good enough for a 2nd inclusion)
-Falco (For being a second Fox, but also because Sakurai could be focusing on more MAIN protagonists)
-Pokemon Trainer (Maybe too problematic to implement, too many characters to spend on one franchise)
-Zero Suit Samus (Could be left out due to time constraints like Mewtwo was)
YELLOW status:
-Lucario (Perhaps dropped in favor of a new-gen Pokemon)
-R.O.B. (Dropped to represent a character from a Nintendo videogame instead of hardware)
I know some of my reasons are stupid, but I wouldn't rule anything out. If it were up to me, they should all come back.