I'mma break things down by existing series
Mario: Most, if not all of the big characters are now in but it's Mario so who know what the limit is
(Captain Toad, Pauline, Daisy, Waluigi, Geno)
Zelda: Depends on if they start integrating characters who were around for one game or played minor roles
(Impa, Midna, BotW Champions, *groan*... Tingle)
Pokemon: Definitely getting a new character, question is who
(800+ of the things, take your pick)
Donkey Kong: Full of potential characters from the across the series.
(Dixie Kong, Cranky Kong, Donkey Kong Jr., Pauline [again] King K. Rool)
Metroid: Has a few choices but most of them are dicey on grounds of size, number of appearances or being in the sub-series of suspect canonicity
(Dark Samus, Stylux, Federation Trooper, RIDLEY)
Kirby: Has a couple of characters but none who've had a big story role. Closest is Bandana WaddleDee being player 2-4 in Rainbow Curse
(Bandana Waddle Dee, Prince Fluff, Gooey, Rick the Hamster, Galacta Knight)
Star Fox: Has a large cast to pick from, just need to hope the series damp performance doesn't impact it's Smash prospects.
(Wolf [again], Krystal, Slippy, Peppy, Chickenwalker Arwing)
Yoshi: Err... Tricky one...
(Yarn Yoshi, Kamek, Poochy?)
F-Zero: Not lacking in characters to pick from, just a reason to do so
(Samurai Goroh, Jody Summers, Black Shadow, other racers)
Earthbound: Ness and Lucas borrow their powers from their allies so that may hamper their chances.
(Ninten, Paula, Jeff, Poo, Kumatora, Duster, Boney, Masked Man)
Fire Emblem: Not lacking in characters but how will the roster be defined?
(Lyn, Chrom, Black Knight [naming but a few], FE Switch protag)
Retro: Likely but hard to determine who
(Ice Climbers [again], Excite Bike, Urban Champion, Takamaru)
Wario: Not lacking in characters but will one make the jump to playable?
(Waluig [again], Mona, Ashley, Kat & Ana, 9-Volt, Captain Syrup)
Pikmin: Nearly had a second character in the last game. Will Alph get his chance again?
(Alph, Louie, Brittany, Charlie)
Kid Icarus: I think Dark Pit is up for a decloning before any new characters
(Viridi, Magnus, Medusa, Hades)
Animal Crossing: There's a lot of aspects to AC that villager doesn't quite encompass. Maybe one of the NPCs will get a chance
(Tom Nook, KK Slider, Isabelle, Ressetti)
Punch-Out: Everyone knows the opponents are the best part of the game...
(King Hippo, Great Tiger, Glass Joe, Mr Sand Man, Bald Bull)
Xenoblade: The series really took off since 4 with two huge RPGs
(Rook [XCX PC], Elma, Rex & Pyra)
Wii Fit: ................................................................... I got nothing
(nothing, nada, zip)
Splatoon: The typical Inklings seen in the debut Trailer are packing the standard Splatershot but there's a lot more the game has to offer
Characters: (Callie, Marie, Cap'n Cuttlefish, Pearl, Marina, Agent 8)
Weapons: (Roller, Charger, Bucket, Splataling, Inkbrush, Dualies, Splat-brella)
Phew... and not not even touching on 3rd parties and new series.