For the halibut, and because @
Burruni
's post was both insightful and entertaining, this is my "who's viable?" list per series:
Captain Toad (i.e. Toad by proxy) is the only "big name" character left in the main series, I think. Waluigi and Daisy are potential candidates, but I can't see either happening unless they appear in a mainline title. Paper Mario could be a potentially viable left-fielder.
Kamek at a stretch, I guess?
Captain Syrup is the only named recurring character aside from The Wa himself, but that's very unlikely.
Ashley seems to be the most popular choice, though her status as an AT scuppers her chances. After that, it's a toss-up between Mona, Jimmy T and possibly 9-Volt.
Dixie and K. Rool. No reason we can't get both!
Impa seems to be the most likely recurring character. Ganon is a possibility, and I think Tingle would be a good candidate if not for his status as an AT. Other popular choices, like Ghirahim and Midna, in addition to being ATs, will be in a good position if they appear in future games.
So much to choose from, yet very few particularly stand out. Aside from a reworked Pokémon Trainer and possibly Squirtle and / or Ivysaur, this is pretty much a case of picking your favourite and sticking with it.
Bandana Dee, as he's become the fourth hero in recent games. Magolor ought to make another mainline appearance before he's properly considered.
Ridley is the top dog here, but that ship has long since sailed, and Dark Samus is the best runner-up. Out of those remaining, I reckon Rundas or Sylux have the best chance, but that's not saying much. Adam or Anthony would be up there, if only by default, but they're not exactly fan favourites.
Toss-up between Viridi, Medusa and Hades - pick your favourite. Unless a supporting character like Phosphora or Arlon makes a huge impact in a future title, a main antagonist seems like the best choice. Magnus, in addition to being an AT, doesn't really do much in the first place. Decloned Pittoo seems the most likely right now.
Wolf and Krystal. Anything beyond that is superfluous.
An antagonist, maybe? I don't think anybody stands out as much as Cap.
Roy's a popular veteran and almost confirmed at this stage. Tiki and Anna are recurring and popular characters, thereby putting them above 90% of the cast by default. Not sure if appearing in a sequel counts as recurring, but let's consider the Black Knight and Micaiah as very low-key contenders. If the new game does well, Kamui may have a good chance.
Doc Louis? This is pretty much the extent of the series.
Porky is the most important recurring character in the series, seems as good a choice as any. The Masked Man has some decent fan support and Ninten would make an interesting alt. That said, the chances of any of them getting in are very slim.
Again, this is the extent of the series.
Ibid.
Tom Nook or Isabelle, as they're both important to the "story"; Nook is a recurring character and Isabelle is very popular in spite of her thus far single appearance.
Metal Face More than likely another hero, pick your favourite.
(retro) Ice Climbers if they can be fixed. Takamaru has a very good chance if he gets another game, and beyond that, there are a few to choose from, e.g. Mach Rider and Lip. I don't think any of them are too likely, though.
(third-party) Snake is in a good position by virtue of being a veteran. Ryu is pretty much confirmed. As for another third-party character entirely, I think Rayman has the best chance, owing to his foot already being in the door and huge fan support.
Pantaloonus Giganticus as a final smash would be really cute and funny!
