I am gonna do it by franchise because I like that format. Please note my ideal form of smash is several separate franchise-focused fighting games released that give you access to make your own roster of 150 fighters from any of the ones you own on a "Mass Smash" online game.
Mario: Waluigi, Toad, Fawful, Geno, De-Alt the Koopalings, Koopa Troopa, Hammer Bro, King Boo, Gooigi, Mallow
Zelda: Vaati, Impa, Skull Kid, The Oracle Pets (at least as mountable items on a stage.), The champions, Veran, Onox, Ghirahim
Donkey Kong: Dixie, Lanky, Cranky, Lord Fredrik, Dredd Kong
Kirby: Bandana Dee, Gooey, Adeline, Dark Matter, Galactica Knight, Taranza, Magalor
Yoshi: Poochy, Kamek, Tap Tap The Red Nose
Wario: Demon-Mask, Captain Syrup, Shake King, Count Cannoli
Metroid: Sylux, Generic Space Pirate, Mini Kraid, Weevil
Star Fox: Krystal, Andross, Dash, Leon, Panther, Grippy
Pokemon: (I didn't think this throoooough) Gengar, Marshadow, Mimikyute, Cranidos, Joltik, Sceptile, Meowth, Obstagoon, Houndoom, Eevee, Galarian Wheezing, SirFetch'd, Reuniclus, Slaking, Tyranitar, Decidueye, Machamp, Hitmontop, Team Rocket Grunt
F-Zero: Samurai Goroh, Pico, James McCloud (alt/Echo of Fox), Black Shadow, Jody Summer
Mother: Pu, Porky, Masked Man
Kid Icarus: Magnus, Phosphora, Pyron, Medusa, Viridi
Splatoon: Octoling, DJ Octavo
Fire Emblem: Arvis, Sigurd, Leaf, Lyn, Hector, Ephraigm, Multi-General (Arden, Wallace, Wendy, someone else), Multi-Mage (Levin, Pent, some others) Multi-Thief.
Animal Crossing: Tom Nook, KK Slider
Fitness: Ring Fit Trainee (can be played with ring con), Drageaux
Punch Out: Doc Louis, King Hippo, Mr. Sandman, Glass Joe
Pikmin: Brittney, Louie
ARMs: Lola Pop, Byte and Barq, Misango, Dr. Coyle, Max Brass (or just do the whole roster.)
Retrotendo: Mike Jones, Baloon Fighter, Mach Rider
Unsaid Franchises: Ryo (Waverace/1080), Lark/Nester (Pilotwings/Nester's Funky Bowling), Harry (Teleroboxer), Mike Jones (Star Tropics), Zoda (Star Tropics), Bubbles (Clu Clu Land), The Amazon (Pro Wrestling), Ice Hockey Player (Ice Hockey).
Okay, gonna stop here before I come up with 20 more.