The original 12: Mario, Donkey Kong, Link, Samus, Yoshi, Kirby, Wolf, Pikachu, Luigi, Ness, Captain Falcon, Jigglypuff.
Main First Party Heroes/Heroines/Villains: Peach, Bowser, Zelda, Ganondorf, Meta Knight, Pit, Olimar, Palutena, Shulk, Inkling, Ridley, King K Rool.
Newest First Party Characters: Isabelle, Incineroar, Piranha Plant.
Fire Emblem: I'll leave this to the Fire Emblem fans, but I would guess Marth and Ike would be high up on the list. Sakurai would likely want to keep as many Fire Emblem characters as possible.
Pokemon: Same with Pokemon, and while I prefer all Gen 1 coming back, it would be better objectively to have generations semi-balanced, which means Lucario, Greninja would be musts. Though, I have a fondness for Pokemon Trainer's unique position of having 3 Pokemon in one.
Retro Classics: Feel like they're all musts: Ice Climbers, Game & Watch, R.O.B., Little Mac, Duck Hunt.
Mii Fighters: While a lot of people don't like them, it's an easy way to make customizable characters of those not in. Would like to see them revamped in some way to allow that even more, perhaps even making them custom for Smash rather than Miis.
Third Party: As many third parties returning as possible. They're the hardest to get back, and so it would be important to bring back as many as possible, with the recognition that especially in this category, not everyone would be able to return. Especially Cloud, is likely to not return. Sonic, Mega Man, and Pac-Man are perennial mainstays. Sega (Bayonetta, Joker), Capcom (Ryu), Microsoft (Banjo & Kazooie), and SNK (Terry) would likely be the easiest to work with. Less certain about Konami and Square, depends on how things are when that time comes. Even Microsoft may be less friendly depending on how the next generation goes. But this is certainly the category where there will likely be far less than desirable returning. Perhaps even less than Ultimate's returning third party cast, since it was primarily feasible due to the "Everyone is Here" plan.
Echos/Doubles/Clones: After all of those are considered, have as many come back as they won't take as much to balance.
I should mention that these are, of course, not the considerations Sakurai would have, but is how I categorize it. It's also not my personal list, as I would add some of my favorites (Bowser Jr.) and get rid of my least favorites. Tried to be as objective as I could be. Also, I'm almost sure I've missed some important ones in some categories, and there were a lot of edge cases where some type of subjective decision had to be made. And an issue comes up when doing this exercise is that it's really hard to make cuts. Whenever I'm given a max, I undoubtedly have to cut characters I think should still be in. Even trying to be as conservative as possible, this has 48-60 characters. It feels like, at that point, get everyone to return. But we know that's unlikely for the next game, and that means a lot if not most of these characters would also be cut. Again, not an enviable task. Which is why I want Ultimate to go on for as long as possible.