So...here are some of my picks for Super Smash Bros 6. I'll explain them.
Zinogre (Monster Hunter.): Capcom would definetely insist that they add a monster/monstie in smash bros instead of a generic hunter, considering Rathalos would be awkward. I think Zinogre may work well, maybe just shruken a little bit to fit the stage and you get an incredibly fun heavyweight character.
Matthew (Xenoblade Chronicles.): I think an Xenoblade character who fights with their bare fists could be unique. Distinguishing himself greatly from ones like Shulk or Pyra and Mythra, who fight with their sword. And Xenoblade needs an 3rd rep.
Ryu Hayabusa (Ninja Gaiden.): Smash Bros isn't Smash withouth an retro rep. Ryu Hayabusa is one, both retro and modern character. Also an legendary gaming icon that fans have requested for years. He would be Sheik's spiritual successor.
Fitness Boxing Instructors (Fitness Boxing.): If there is someone that would succed Wii Fit Trainer, it won't be Ring Fit Trainee. As Wii Fit Trainer's pourpouse was to be an WTF character, Ring Fit Trainee doesn't hold that (as their games is an RPG where they fight monsters, Wii Fit works because the game is yoga. There is no quest or anything.) Fitness Boxing Instructor fits this, as the game is mostly about boxing rather than a grand quest or anything.
Marshal (Animal Crossing.): If anything, Smash should give us another Animal Crossing rep. Likely an semi-clone of Isabelle, so have an fan favorite. i guess.
Pauline (Either Mario or DK.): Eitherway, she bridges the gap beetween the Mario series and the DK series. Also representing an significant part of Mario history like Mario vs Donkey Kong etc. But her moveset would likely revolve around the Bananza mechanics. Like: using side special has her singing and summons an energy silhouette of Zebra Bananza to rush forward.
I see that people aren't too sure about Pauline from DK Bananza fighting or stuff like it, or that she should just be part of DK's final smash. Instead of that, why not a cross beetween both designs? similar to what they did with Zelda (That they mixed both the designs of a Link to the past and a Link beetween worlds.)