Shovel Knight, Shantae, Edea Lee (or someone else from Bravely, maybe with a job switch option?), Bomberman. Maybe a generic Monster Hunter too, with Palico as an assist trophy.

Snake? Snake? SNAKE?!?!?!?!?!?
Also, I'd love to play as Chun-Li with the same style of controls as Ryu.
I also want Ryu Hayabusa. And Raiden. And Dante.

Wait, are you in Gamefaqs?

Actually, SSBM was Roy's debut game. I wonder if they'll ever do that again.

I wonder if we could get Sora in there as well. That would be great. We got some nice suggestions here.... very plausible too!

I want King Boo.

Crono and/or Geno...
Jack Frost from Shin Megami Tensei...
I think we can assume inklings will be there...
Ice Climbers better be back...
And since every game seems to reintroduce at least one old school forgotten character, I'll say Mach Rider...
And I know this will probably be a very unpopular opinion, but I found Pokémon Trainer to be a lot more fun than Charizard, so I would love to see Ivysaur and Squirtle again...

I just want Bomberman... T_T

Elma, with Ares 90 final smash.

I think Inklings are pretty much a given, due to Splatoon's popularity. I would also like Sceptile since there is already a fire and water starter, and sceptile is my favorite starter of all, so I'm biased. Girahim and/or Skull Kid would be neat additions since they're probably the best Zelda villains besides for Ganon.

-Blaziken
-Litten's final evolution (if the leaks are true)
-Inklings
-The Final Boss from Splatoon (not gona spoil even his name to people)
-Ashley (Wario Ware)
-Rhythm Heaven characters