Certainty
Death, Taxes, &
: I think this will either be a Gen 10 starter, or Eevee if one isn't available. Eevee seems like the most likely pick for them to go back for, and since Pokémon Scarlet/Violet is old now, and Pokémon Legends Z-A is a spin-off, I don't see them adding a character from either game just for promotion's sake. Gen 10's going to release very close to SSBNext, so I'll give the edge to that outcome, but something else is actually a possibility for once.
Highly Likely
: Waluigi is the most likely, but for one reason or another, he could get snubbed for (Captain) Toad or King Boo.
: I am biased, but with the success of Kirby and the Forgotten Land, I think it's time for Bandana Waddle Dee to take the stage.
: I think Tom Nook is just shy of being a lock. I also think he's not going to overlap with Villager and Isabelle and instead offer something entirely different.
: I think Splatoon will be getting a non-echo fighter, even if it's Octoling. Octoling will be like the Lucas to Inkling's Ness. Alternitively could be D.J. Octavio or an idol, but Octoling seems more likely.
Plausible
(Geno): If Geno is added, he will probably be a second Mario character rather than being the only one.
: It's been a long time since the series has gotten a character, but barring a surprise Skull Kid, there aren't too many compelling options. Rauru is possible, but not typically the kind of character they'd pick, and it's more likely they'd just change Link and consider The Legend of Zelda: Tears of the Kingdom repped.
: I feel like Sylux is a possibility to promote Metroid Prime 4, but it's also very possible they don't bother for a lot of reasons, not the least of which being power fantasy overlap with Samus and Dark Samus.
: I could see Kamek happening, but being coined as a Yoshi character, as this is his debut, though he's really just operating as one of the less likely options for a Mario character here.
: WarioWare is still going pretty strong, so Ashley is an option. Not the most likely, but one I veer more towards the side of optimism on.
: There's a lot of different options they could pick here, but any one of them could happen.
: I think Pikmin is poised for a newcomer, and I'm still optimistic on Oatchi, but I cant deny the possibility of them just giveing Olimar a new costume and calling it a day.
: Chun-Li is has a pretty decent shot at getting added. Luke also does, kinda, but I'd say Chun-Li's chances are much greater.
: Jin is a pretty plausible pick, either being added alongside or instead of Kazuya.
Implausible
: They could try to go back and add Lyn, but I don't think they're super likely to do this.
: Zero could be added, but I'm not super optimistic on them doing so.
: With Noah/Mio no longer being promo picks, it's pretty likely that Xenoblade Chronicles 3 will skip out on a character. Maybe Xenoblade Chronicles 4 will have a shot in the DLC? Crap, depending on how things go Pyra/Mythra could be the last Xenoblade Chronicles characters for a good long while.
: Not unless we're replacing Simon and Richter with Alucard.
Nope