So here's a little game.
Disregard how likely you think the characters are.
Tell us how interested or satisfied you'd be with the character if included.
Incineroar
Ken
Isabelle
Geno
Isaac
Monster Hunter
Lloyd Irving
Dixie Kong
Impa
Skull Kid
Black Shadow
Bandana Waddle Dee
Ninten
Diskun
Raiden
Big Boss
Shadow
Elma
Rex and Pyra
Jeanne
Spring Man
Wonder Red
Incineroar: I haven't played much of Pokemon beyond gen. 3 (I've played a bit of Gen. 6, but that's about it), so I'd have no strong feelings one way or the other
Ken: I'd be fine with it; he makes sense as an echo fighter, and while he's not my favourite Street Fighter character, it would still be fun to have that alternative to Ryu
Isabelle: I've had a tough time sticking with Animal Crossing (mainly play the Gamecube one for the NES games), but I feel like she could be entertaining
Geno: he was my favourite party member in Super Mario RPG, and that was one of my first games, so I'd definitely be excited.
Isaac: I haven't played much of Golden Sun, but I'd definitely be interested in seeing what they'd do with him
Monster Hunter: meh. I didn't enjoy what I've played of Monster Hunter (Tri, for the Wii), and there are probably about half a dozen Capcom characters that I'd prefer instead.
Lloyd Irving: he could be fun to play as. It might prompt me to finish Tales of Symphonia.
Dixie Kong: while I preferred K. Rool, getting both of them in would be great.
Impa: I would like to see her included, especially if it's the Hyrule Warriors version
Skull Kid: outside of Midna and Ghirahim (who are already assist trophies), I feel like Skull Kid likely has the most material to work with for a new Zelda character; I'd definitely be excited to see him included
Black Shadow: as far as F-Zero antagonists go, I'd prefer Deathborn; his design's better in my opinion.
Bandanna Waddle Dee: I'd definitely be happy to see Kirby get a new character.
Ninten: I'm not sure what he brings to the table that Ness doesn't already have. I'd be okay with a third Mother/Earthbound character, but I'd rather see a villain like Porky, Masked Man, or the Mother 1 version of Giygas.
Diskun: no idea what he can do.
Raiden: depends (I'm assuming you mean the Metal Gear one). If it's the MGS2 version as a Snake echo, that wouldn't be very exciting. If it's the MGS4/Rising version as a unique character, I'd be super-excited.
Big Boss: I'd definitely be fine with him as an echo. And if it means that we get a trailer with the Snake Eater theme? Yes, please.
Shadow: assuming you mean the hedgehog, as opposed to the Final Fantasy 6 ninja. I'd be okay with him, but I'd prefer Eggman or Tails as a second Sonic character.
Elma: I'd definitely be excited - I've really enjoyed what I've played of Xenoblade Chronicles X, and I'd hate for it to be forgotten due to appearing on a less popular console
Rex and Pyra: based on chapter 1 of Xenoblade 2 alone, I could totally see them fitting in Smash, and I'd like to see what they could do with the Blade/Driver dynamic.
Jeanne: I definitely wouldn't mind seeing it happen, in spite of the undoubtedly negative reaction from the competitive community. Jeanne would make perfect sense as an echo/clone, since that's how she plays in her source material, and Bayonetta 2 was one of the main reasons why I bought a Wii U.
Spring Man: I haven't played Arms. I might be a little interested, but Spring Man seems like the least interesting character in the entire cast.
Wonder Red: it's the closest we'll get to Viewtiful Joe in Smash Bros., and if it means that the Wonderful 101 gets more attention, then I'm all for it.