Mario, Luigi, Peach (Daisy echo), Yoshi, Bowser, Wario, Ashley, Donkey Kong, Diddy Kong, Olimar
Link, Zelda, Ganondorf, Impa, Marth (Lucina echo), Byleth, Alear, Shulk, Aegis, Samus
Villager, Isabelle, Tom Nook, Kirby, Bandana Waddle Dee, King Dedede, Meta Knight, Ness, Captain Falcon, Fox
Pikachu, Charizard, Mewtwo, Lucario, Tinkaton, Min Min, Ring Fit Trainee, Little Mac, Pit, Isaac
Inkling (Octoling echo), DJ Octavio, Ice Climbers, Pac-Man, Sonic, Cloud, Sora, Steve, Mii
Reasons
I’m treating this as a pure reboot, so I think it’s harder to justify Yoshi with his own franchise.
I think Wario Ware is popular enough to bring in Ashley.
While the triforce would honestly work well enough here, I added Impa as the five fighter limit severely restricts Mario, and I can’t really use Sheik or a child Link here, and Impa would be based on Age of Calmity.
Villager and Isabelle are in Mario Kart 8 DX, and Tom Nook is also prominent.
DJ Octavio provides an antagonist.
Pikachu, Charizard, and a promotional pokemon feel kind of mandated. The only other thing that I’d want is a Pokemon Trainer, but I’ll assume a fully fledged trainer is not allowed, so Red will cheer Pikachu in the background. If extra fighters were allowed, they’d probably be Mewtwo and Lucario, with Mewtwo probably being the larger priority.
The worst case scenario for Kirby is that we lose Meta Knight, and Bandana Waddle Dee is added given that the latter seems to be the player two character.
Fire Emblem is complicated in a pure reboot scenario, but I’ll have Byleth and Alear here, and Byleth would be a sword/magic hybrid fighter. For the last fighters, I was conflicted between Marth and Chrom, and Marth barely wins out here.
In terms of Xenoblade, Shulk and Aegis are perfect as they are with both the Monado and transformation gimmicks. I guess you could argue for Noah or Mio, but Cross/Elma is missing too, so I’m fine leaving Noah/Mio out.
Ridley is absent as he’s not in Dread, and SSB is being kept to a minimum here.
I don’t think there’s much to say about Pikmin.
ARMS and Ring Fit Adventure are the Switch’s biggest original games.
At this point, I’m probably must going to either further supplement some franchises or bring in some obscure ones. In terms of the obscure ones, Punch Out!!, Kid Icarus, F-Zero, Star Fox, and Mother were the most prominent ones. I think that Ice Climbers and Isaac are cool ideas from a fighting game stand point, so they join as additional deep cuts.
For third parties, this is really just a cross of what I‘d want and what Nintendo would want. While the 7 remasters were Playstation exclusive, they do have older Final Fantasy games on the Switch.