I'm going to be the devil's advocate:
Aside from at least 3 newcomers at E3 and 2 veterans at some point before and/or after, we've seen the last of pre-release reveals, assuming unlockable characters are still a thing. That would leave us at 30 characters for the starter roster (31/32 if Sheik and Zero Suit Samus are back. Also assuming Mega Man and Sonic are starters, as well as Luigi). That seems like a number that makes sense. I think anymore than that would hint at unlockable characters being a thing of the past.
As for who I think they will be?
Palutena for our next newcomer, possibly before E3 (hopefully really soon).
Professor Layton and Chrom for E3.
As for veterans, Snake, Yoshi, and Ice Climbers are the only ones I see being revealed pre-launch, though not necessarily all of them at once. Maybe Ike, if he returns.
Other than that, any other potential characters are unlockabe.
The reason why I think the above characters will be revealed is because they are ones people have really been looking for all over the world.
This is me assuming there aren't any new Metroid games or F-Zero games coming at E3 or anything like that.