Rosalina & Luma: Could work
Bowser Jr: YES, been a fan of him since his debut in Smash 4
Dr. Mario: Nah, I don't think it'd be worth editing all of the sprites to get a really similar clone.
Geno: All of my yes, the movesets in Crusade and Legacy XP are kinda disappointing and I'd be down for an actual Geno moveset
Paper Mario: Not too sure about this one, but it could work

Ganondorf: Yes, although i'd rather see an original moveset and with another design
Toon Link: Could work
Young Link: Only if the masks are added as part of his moveset (and FS being Fierce Deity)
Skull Kid: Yes please

Dedede: Yes

Diddy Kong: I'd like it, tho i'd be content if K. Rool makes it in instead of him
K. Rool: YES, maybe a few moveset tweaks from the Ultimate one (like Down B spawning a kremling or something like that instead of counter, which i think could be moved to Side Taunt)

Ashley: Maybe

Charizard: Nooooot too sure about this one
Lucario: Maybe
Mewtwo: Definitely would like him in
Greninja: Another maybe
Sceptile: Probably my favorite pick, I'd love more Pokémon newcomers in fangames

Dark Samus: Literal palette swap, so I think it'd work fine. If she gets some custom moves samus had in Smash 4 plus Phazon attacks I'd love it even more.
Ridley: Yus

Snake: Yes

Wolf: I'd love him in but I don't see him likely

Knuckles: Yes
Shadow: I'd love him but I don't see him likely

Lucas: Yes

Palutena: If some of her moves are replaced with her custom moves from Smash 4 then yes
Dark Pit: He's a literal palette swap of Pit, so I think it'd be good to add him. Tho, using some of his custom moves from Smash 4 as specials. Also using his staff in more attacks.

X: Absolutely would love him in, there's plenty of potential for a different moveset given he's got 8 games to work with.
Zero: Yes

Kratos: Not too sure

Crono: I'd love him in, but mainly if he uses his partners in attacks instead of relying in swordplay and magic (cause Isaac already covers that)

Cloud: I'd like him in.

Duck Hunt Duo: Maybe

Renji, Vegeta, Sasuke, Zoro: These 4 would be cool additions.

ROB: Maybe

Shulk: Maybe

Villager: Maybe

Olimar: Yes

All the FE dudes in the picture: I think only two out of these would be good, either Robin, Corrin or Ike

Nights: All of my Yes

Ice Climbers: Yes

Crash: Maybe

Ryu, Evil Ryu & Chun-li: Not too sure

Bayo: Nah

Shovel Knight: I'd like him in

Klonoa: Yes

Alucard: Absolutely would love him as the 2nd Castlevania char
Soma Cruz: Ehhhhh

Shantae & Nega: Maybe

Spring Man: Yes

Inkling: Yes

Firebrand: I'd love to play as him, legitimately my favorite pick in the picture.

Chosen Undead (I think that's his name?): Nah

Saki: I don't think you can do much besides what Crusade did

Blade & Blue: Nah. Devs already deconfirmed any OCs

Little Mac: Maybe

Mach Rider: He'd be cool but I don't see him as likely

Sash Lilac: She'd be fine

Yeah, Jam & FURYYYY: These kinda count as OCs, regardless i'd main the three so Yes