I think the wide gap between Ultimate and Smash 6 will help to give us a bigger pool of newcomers to consider, which should lead to some interesting speculation.
At the moment, the most likely additions would be:
Pokemon (obviously)
Spring Man or Min Min (assuming ARMS sticks around)
Fire Emblem (probably)
Xenoblade (which is one big question mark that depends entirely on where the series goes next)
And I'd argue that Mario and Kirby could potentially get another rep each, with King Boo probably being the front runner for Mario considering that Luigi's Mansion is its own subseries now, and then Bandanna Dee, Marx, or Magolor would all work fine for a fourth Kirby rep (though with Sakurai still calling the shots, anyone besides Marx would probably have really poor odds).
After that is when things get murky:
Splatoon is big enough to get another rep, but it doesn't really have much to work with beyond Octolings. They could work as a new character using Splatoon 2 weapons, but I still feel like they'd be kinda samey compared to Inklings in some ways.
Zelda already has all of the important characters that aren't one-offs. For it to get a newcomer, I feel like either an old character would have to be brought back with an incredibly substantial role, or a MAJOR new character would have to be introduced.
Dixie Kong seems like the biggest missing echo/semi clone. She wasn't popular enough that the Smash Ballot could do anything for like it did for K Rool, Dark Samus, and Chrom, so I really can't see her being unique, but she'd work just fine as an semiclone like Wolf or an echo in the vein of Chrom.
After that, everything for first parties pretty much just hinges on what games come out and gain popularity in the coming years before Smash 6's project plan. I wouldn't mind a roster with similar additions to Smash 4's, with interesting choices largely hailing from newer games.
