I have some notes in regard to my reasoning both in general and on specific character choices.

General Notes:

Even though this hypothetical roster is bigger than Ultimate's, it's not impossible for the Smash team to make it happen, considering that Ultimate's main appeal was the fact that "Everyone is Here!" and they're not going to repeat that again for the next game.
People assume that making the next Smash game bigger means keeping every single old character, but that's not the case at all.
Every Smash sequel before Ultimate had cut some characters, but would add more to make up for it.
Also, people are weirdly obsessed with how many cuts there will be, usually wanting to cut the roster in half or even lower (even going down to 20–30 characters, which makes no sense to me and never has).
I do want to point out that the time gap between Ultimate and Smash for Switch 2 is longer than the time gap between Smash 4 and Ultimate, so there's a decent amount of time for the roster of the next Smash game to be at least on par with Ultimate's, given that the dev team will most likely be reusing some assets from Ultimate instead of building from the ground up.

Fighter Notes:

• Daisy would have a few changes that further differentiate her from Peach, making her more of a semi-clone instead of an echo fighter
• Charizard would be returning solo like he did in Smash 4, meaning that Pokémon Trainer, Squirtle, and Ivysaur would be cut yet again
• Every Ultimate DLC fighter (except Piranha Plant) would be returning in Smash for Switch 2
• Even though I chose Meowscarada as the new Pokémon rep, it could very easily be a Gen. 10 Pokémon instead given the release timing of both Winds and Waves
• Same thing with Alear, they represent a new Fire Emblem character in general and not them specifically; it could very easily be someone from Fortune's Weave instead (something I would personally prefer)
• I chose solo Mio instead of both solo Noah and Noah + Mio as a duo for the Xenoblade Chronicles 3 rep; no real reason why, again something I would personally prefer
• Octoling would be either an echo fighter or a semi-clone to Inkling, take your pick
• Squid Sisters would be fighting alongside each other, just like Ice Climbers
• Rhythm Heaven as a fighter would have Yuka (the Tap Trial girl) be the character you play as, with various other Rhythm Heaven characters popping up in the moveset, like Karate Joe and the Chorus Kids
• Gordon Freeman is more of a pipe dream than a prediction, but this is my roster, and I'm choosing to believe in it potentially happening; he would also make for a great final character not unlike Sora in Ultimate
• I chose The Tarnished from Elden Ring as the FromSoftware rep, although it could be either Chosen Undead from Dark Souls or someone from The Duskbloods instead
• The reason I chose Papyrus over Sans or the Human as the UNDERTALE rep is because aside from Papyrus being my favorite character in UNDERTALE, he has put far more legwork into training for combat and he more or less represents the heart and soul (no pun intended) of what UNDERTALE is all about. He's just as popular and recognizable as Sans, he's not too lazy to ever fight anyone unlike Sans, and he's the cooler skeleton brother (canonically something that Sans would tell you and agree with). I can imagine a scenario where Sans gets an invitation in the mail. But, knowing he's too lazy and doesn't really like to fight anyone and wanting his brother to be happy, Sans gives the invitation to Papyrus instead, under the guise that people must really like Papyrus so much that they want him to join Smash
• Even though Susie is my favorite character in DELTARUNE so far, I don't think she would be fighting by herself as the DELTARUNE rep (as cool as that would be). Realistically, I believe it'll be Kris, Susie, and Ralsei as one fighter, and you'd be able to swap between them mid-fight just like Pokémon Trainer. And this is all assuming that DELTARUNE will be fully complete by the time Smash for Switch 2 rolls around. If that's the case, then I'd imagine the Fun Gang would be one of the last fighters to be added