By the way, let me be unpleasant. I think considering Wii Fit Trainer fairly safe in the face of abundant cuts denotes that you guys really underestimate what a heavy cuts scenario would look like.
At that point you can't just consider characters in isolation, there's also the opportunity cost of keeping one character in place of one or multiple others, not to mention in relation to newcomers.
So here it is, my (mostly?) rational and unbiased cut list.
Dr. Mario - has been cut before, he's dead
Rosalina - could get cut, Nintendo doesn't use her as much anymore and I imagine she's one of the most labour-intensive fighters
Bowser Jr. - cut for sure, 7 alts is a ton of work
Young Link - he's probably not going to be prioritized over Toon Link
Zero Suit Samus - with other female action heroes now in the game, and Ridley working much better as the 2nd Metroid fighter, I think she's dead
Dark Samus - got lucky enough to get in one time
Wolf - Falco has already survived over him from Brawl to Smash 4
Jigglypuff - Sakurai will finally manage to give her the boot at the 6th try
Pichu - old news and ultimately a clone
Squirtle/Ivysaur - already gone before
Greninja - popular but not current and doesn't play as unique as Incineroar
Lucas - Mother 3 will never release in the West
Roy - semiclone, already cut before
Chrom - goes with Roy
Ike - Cloud and Pyra play similarly but have more gimmicks going on, plus they're both more popular
Corrin - as sure of a cut as sure can be
Ice Climbers - harder to bring back compared to other wacky characters
Palutena - Kid Icarus is dead
ROB - I feel like they'd prioritize Mr.G&W and Duck Hunt over him
Wii Fit Trainer - replaced by Ring Fit, as the roster shrinks there's less reason to keep similarly-themed characters in; and she's not in the Ness/Captain Falcon tier of untouchableness
Shulk - if cuts get heavy I don't see him surviving against Pyra and a new Xenoblade character
Snake - probably an expensive license, already got cut before, Duck Hunt and Pac-man are projectile-based trappers too
Bayonetta - if there's no Bayonetta 4 in the works and cuts get heavy, she could get the boot
Hero - probably too much of a pain to license for base game
Steve - a nightmare to develop, and probably also to license
Banjo & Kazooie - they're popular, but not the easiest to license 3rd party characters I think and there's always K. Rool for Rare nostalgia
Terry - just a fairly likely cut overall
Honorable mentions, characters who are at risk but I think are more likely to stay:
King K. Rool - I believe they're only going to keep him due to just how well-received he was a fighter, he might probably be the most popular Ultimate newcomer in general
Mii Fighters - Nintendo is phasing the Miis out, and it's three not super unique movesets to develop. I think they're only going to be kept around to sell DLC costumes
Joker - people just went crazy for him and Persona 5 is everywhere these days, probably not as high priority as other 3rd party characters bur I think they'll try to keep him
Sephiroth - Cloud won't get cut and at that point they'll just make the two of them a package deal
Piranha Plant - way easier to develop than both some other Mario characters and other wacky characters, a great encapsulation of how wacky Smash can get; I think Plant stays
Sora - could go either way but I don't think he'll get cut
Diddy Kong - DKC is dead, I don't think he's played that much outside of competitive play... But he's one of Nintendo's icons, he has to stay
Olimar - mostly same reasoning as Diddy
Villager - also same reasoning; but it push comes to shove I feel like they'd keep Isabelle
Falco - really hard to see this guy go, he's super popular in the Smash community, but ultimately he's a semiclone from a dead series
Sheik - similar to Falco
Now, do I think getting all these cuts would suck? Absolutely! I'd just keep playing Ultimate to be honest...