This priority list will assume that SSB never existed.
Must be in (Treating as 14, but technically 16 since the Miis are separate fighters)
Mario, Pikachu, Link, Kirby, Yoshi, Villager, Inkling, Chrom, Donkey Kong, Luigi, Charizard, Samus, Shulk, Miis
I’d add in a Brawl sized roster (16)
Bowser, Peach, Sonic, Ness, Olimar, Fox, Pokemon Trainer (by themself), Wario, Zelda, Isabelle, Ganondorf, Ice Climbers, Mega Man, Pac-Man, Steve, Cloud
I’d add in a For sized roster (16)
Diddy Kong, King Dedede, Ike, Lucario, Ryu, Simon, Bayonetta, Pit, Pyra-Mythra, Mewtwo, Rosalina/Luma, Meta Knight, Bowser Jr., Byleth, Min Min, Ridley, Little Mac, Duck Hunt
I’d add in an base Ultimate sized roster (13)
Sora, Byleth, Snake, Hero, Joker, Terry, Sephiroth
Nice extras/clones, they can be added before thise above them (8):
Young/Toon Link, Daisy, Pichu, Lucina, Falco/Wolf, Ken, Richter
Charaters I can’t see my self adding in no order ():
Captain Falcon, Jigglypuff, Sheik, Wolf/Falco, Dr. Mario, Marth, Toon/Young Link, Roy, Mr. Game & Watch, Zero Suit Samus, Lucas, R.O.B., Wii Fit Trainer, Palutena, Robin, Dark Pit, King K. Rool, Dark Samus, Incinoroar, Piranha Plant, Banjo-Kazooie
Reasons
Must be in: most of these represent today’s biggest franchises for Nintendo. Samus is in for being Nintendo’s first successful female protagonist. Charizard is a popular Pokemon that the Pokemon Company likes to use, and Miis are profitable/customizable. It was hard to choose the FE character since Marth is like Captain Falcon and Ness (and Jigglypuff, but it tends be treated as low priority) where he is prioritized for being a staple to SSB, Ike is the popular one, Chrom appears in crossovers and is a protagonist of the series most important entry, and then Byleth comes from the most recent/successful entry. I’ll probably go with Chrom and then the most recent protagonist.
Brawl sized roster: Most of the bigger Nintendo franchises get more fighters. Sonic and Mega Man are popular third parties. Ness and Ice Climbers give us really obscure characters, but Duck Hunt, Little Mac, and Pit would not be very farfetched either. In terms of order, I went with Bowser and Peach first for respectivley being the most notable villain and female from Nintendo, then Sonic for bringing in third parties. Before adding some less popular and relevant franchises, I wanted to add an obscure character with Ness. After Ness, I’d add in the less popular but long lasting franchises with Star Fox and Pikmin. After that, I’d add the protagonist of Pokemon, Wario, the rest of the triforce trio, and a popular NPC of Animal Crossing. I want add one mkre obscure character and then start adding in more third parties after Sonic.
For sized roster: I wanted a couple more Pokemon, Mario characters, and less popular, but relevant fighters. I wanted more third parties. I added a couple obscure fighters. I wanted a couple FE characters. Daisy and Pichu are last minute clones.
Base Ultimate sized roster: I added the remaining obscure fighters, more Mario and FE characters, more third parties, three popular requests, and a couple low effort characters.
Clones: I think that multiple Links are important as Link reincarnates. Daisy is a recurring Mario spin-off character, and Pichu is related to Pikachu. Lucina could be based on Chrom. I could seeing a second Star Fox character being possible as a Fox clone. If time permits, I could see myself adding Ken and Richter
Can’t see myself adding: Even if F-Zero was a well received franchise, I feel that I would’ve come up with adding Captain Falocn when F-Zero was relevant. While these characters are easy to make, I think timing really helped Jigglypuff, a third Star Fox character, and Dark Pit. I would have never come up with the idea of Mr. Game & Watch and Wii Fit Trainer. Since I’d have a bigger focus on third parties, I‘d assume that I’d probably overlook R.O.B. as a playable fighter. I probably would not think of adding three Links. Roy, Palutena, and Corrin would not have the timing to be in, and Corrin is controversial. I like Zero Suit Samus, but regular Samus is enough. Like Zero Suit Samus, Sheik isn’t needed for Zelda’s moveset, and OoT is 23 years ago, so I’d probably just have my LoZ cast as the triforce trio and a child Link. Lucas is unfortunately exclusive to Japan. While she was popular for Ultimate, I’m unsure if I would think of Dark Samus since she hasn’t been in a Metroid game for awhile. Similarly to Dark Samus, I’d probably overlook K. Rool since he hasn’t been in a game for 13 years, and Dixie Kong would be fitting choice for a DKC character. For Incinoroar, I’d probably pick Mimickyu as my Gen 7 Pokemon, but if I’m being honest, the Pokemon on my roster would be Kanto Pokemon, Lucario, advertisements, and maybe Pichu as a clone. For Piranha Plant, I’d rather add characters who are not generic enemies. Since I would prioritize other Microsoft characters, I could not see myself add Banjo-Kazooie. While a clone of Mario would be probable, it would more likely be Metal Mario. I like Marth and Robin, but Marth is mostly known for being in SSB, and Byleth is also in a popular game, and he can use magic in this hypothetical.
and then went back to 74 with 
