I don't think whatever the next Smash Bros game is going to be, it'll have to be a total reboot. It'll just have to have some big selling points - be it a flashy new story mode, a killer newcomer selection or just an extreme amount of polish. Sakurai is a huge overachiever so if he comes back I can't imagine the veteran roster being cut down that much, but its hard to say what will happen if a new developer takes his place.
As for which characters get cut, it's hard to say because all characters have their merits. I think the most likely too go are characters from franchises who already have a lot of characters (
), characters who are difficult to develop (
my main
, though he could be saved by popularity) or clones who are duplicates of existing characters (
). This prediction isn't baseless - traditionally these types of characters have been the only cuts we've seen in Smash. Mewtwo and Roy in Brawl were probably lower priority since their respective franchises had more then one character each, Doctor Mario and Pichu were probably cut for being filler "easy clones" of Mario/Pikachu, and in Smash 4 we saw Icies and PT cut due to technical limitations (granted I don't see technical limitations being a problem going forward, but resource consumption might be).
On the other hand I think fan favorites and characters who are the sole representatives of their franchises will have a little extra buffer protecting them from cuts.
is a cut I often see speculated and I just can't see that happening because he's so popular and is our only F-Zero fighter, even if F-Zero isn't being pushed by Nintendo. He also has a really simple, no-gimmick moveset.
But for the time being, I think I'm going to focus on appreciating the game we've got. I'm a big Pokemon and Fire Emblem fan so I'm not excited to see who from those franchises might get cut. And hey, even if Smash Brothers 6 Featuring Funky from the Knuckles May Cry series has a terribly mutilated roster, I'll always be able to go back to Ultimate when I wind up in the mood for some veteran fighter action. I'm feeling open-minded to whatever a future Smash Brothers game has in store.