Okay, let me try! Unique Newcomers for each series (though my attempt won't be completely based on who I'd want):
Super Mario Bros.: Paper Mario
Yoshi: Kamek
Donkey Kong: Dixie Kong
The Legend of Zelda: Tingle
Metroid: Weevil (from Metroid Prime Hunters)
Kirby: Bandanna Dee
Star Fox: Krystal
Pokemon: Urshifu (could make a great Gen 8 addition for FP2, assuming it's a "loose-ends" pass)
F-Zero: Black Shadow (if he were to get a moveset revolving around his plasma whip)
Earthbound: Porky Minch
Ice Climber: Polar Bear
Fire Emblem: Tiki
Kid Icarus: Hades
Wario: Ashley
Pikmin: Titan Dweevil (though they'd have to shrink it a lot)
R.O.B.: Professor Hector
Animal Crossing: Mr. Resetti (even if he were to play similarly to Isabelle and the Villager, he could still have moves that involve digging, such as to "teleport" to a different spot)
Punch-Out!!: King Hippo
Xenoblade Chronicles: Elma
Splatoon: DJ Octavio
ARMS: Dr. Coyle
Metal Gear Solid: Raiden
Sonic the Hedgehog: Dr. Eggman
Mega Man: Zero (though he'd probably have to be in his MMZ incarnation)
Pac-Man: Maybe Patra? (someone in the Newcomer Speculation area has come up with a moveset for her)
Street Fighter: Vega (since he uses a claw and, if I recall correctly, ninja-like moves)
Final Fantasy: Squall Leonhart (they could probably add a mechanic to his Smash attacks where after you release the A button, you have to press it again at the right time to deal critical hits)
Bayonetta: Balder
Castlevania: Alucard
Shin Megami Tensei: Jack Frost
Dragon Quest: Slime Knight
Banjo-Kazooie: Gruntilda Winkybunion
SNK (okay, I'm gonna cheat a bit here by increasing my range from just Fatal Fury): Marco Rossi
Minecraft: IDK Pillagers (don't know what moveset you could make with just a crossbow)
Super Smash Bros. (for the Miis, since that's the universe they get their emblem from): Tabuu
Ones I don't know about: Wii Fit, Game & Watch, Duck Hunt