Exactly. The problem with hypotheticals like these is one tiny change a long time ago changes everything. Sakurai mentioned during Melee they couldn't do Banjo & Kazooie and James Bond for a variety of reasons despite being well requested, and in the former case being an "obvious" inclusion. And we also know they tried to get trophies for Banjo & Kazooie in Melee, but the problem before the buyout was working with a company overseas. They more than likely would have been a part of Brawl, but assuming they weren't, and that Nintendo/Rare didn't do another Banjo-Kazooie games after the GBA ones, I would see Banjo & Kazooie not being as requested, and would have at best been a retro pick--not very requested, but used as a way to celebrate Nintendo's history, particularly since there's little from the N64 era. Banjo & Kazooie are highly requested in this universe, primarily because the legal issues challenging them being in made them more of an "impossible" pick despite having such a big legacy with Nintendo. Those two aspects, along with the feelings of their new company not doing anything with them despite their potential, added immensely to their appeal.
And in terms of cuts, disregarding any legal challenges, I feel like they would prioritize the older classics (Especially the original 8, but also third parties like Sonic and Mega Man), and the newest characters, particularly DLC, and particularly those who were highest on the ballot. Sora, Banjo & Kazooie, Ridley, King K. Rool, Simon, Min Min, and Pyra/Mythra. Though, I'd also like to see characters like Hero and Kazuya return, even if they're not my personal cup of tea. And I'd hate to see Joker cut, as I love Persona content in Smash. Even though between Fighter's Pass 1 and 2, there were only 3 characters I actually liked, most of them I would say were really significant choices, and should be a part of the base game next time.
Which if they could get over the legal hurdles of getting Sora and Cloud again, I feel like they might as well go for all of them and make an Ultimate Deluxe. And with the DLC now in the base game, they can put more time in them than they could as DLC. For Hero, they even mentioned wanting to make 8 heroes instead of 4. And some DLC could get base game content, such as assist trophies (like Morgana), bosses (like Gruntilda), and items (Shock Bomb). There's just too much great content that could be possible in an Ultimate Deluxe, that would be impossible in a game with significant roster cuts because those series wouldn't even be in the game. Just so Donkey Kong could throw barrels, which he probably wouldn't anyways in a cut roster because veterans not getting new moves is less a budgetary thing, and more a part of Sakurai's philosophy of not changing characters people may have already grown attached to (otherwise there were 3-4 other games he could have changed them in). And why take such effort retooling characters already in the game when a new character could both be fresh and bring those kinds of moves into the game--retooling veterans seems to only be an additional thing with whatever resources are left over. And while an Ultimate DX just delays the inevitable, I say delay the inevitable as long as possible. Because, inevitably, a Smash with a cut roster will disappoint more people than any additions (single player, retooled veterans, new modes) would make up for.
And for me personally, if there's a significantly cut roster, I'd stick to Ultimate. Even if I got characters I want, like Crash, it wouldn't mean much if that meant Sora and Banjo & Kazooie were no longer in, or even characters like Pokemon Trainer, who I've grown attached to over the years. If DLC characters were in the next game, but as DLC again, it would be disappointing, and felt like a waste of an opportunity to do newer stuff with them. And my biggest complaint about Ultimate has been terrible single player content, which sucks because that's the only way I can play it. But if the choice came down to keeping all the characters, including the terrible ones, or getting a good single player mode, I'd pick keeping everyone for as long as it's possible.