If I try to be realistic, I think Nintendo would aim at supporting their big IPs, or their popular/iconic switch games. So from those premises, I would guess, based on first party picks only, that Springman has good chances, because he is recognizable and unique.
What big titles are upcoming, or that have been recently released? I could see something from Luigi's Mansion 3, so Polterpup, Professor E. Gadd, or actually King Boo since main villains tend to make it into Smash too. There's obviously going to be a Pokemon added from the last generation. We have a fully evolved Water (Greninja) and Fire (Incineroar) starter so far, so now it's time for Grass, so one from the Grookey line is my guess (I don't follow Pokemon much, I wonder which starter was the most popular this time around). There's the next Breath of the Wild that's going to be big, so they could release that version of Zelda, provided she plays a bigger role. Or some other iconic character from the series making a come back. Then there's also Metroid Prime 4 on its way, that could inspire a little bit. There's also potentially a Pikmin 4 that has been in the works for years, but has still to be properly announced...
On their older franchises (because they like nostalgia, it's part of the appeal), they could go for Dixie Kong which is super iconic and not yet in there. Skull Boy would make tons happy and is one of the most popular villains in a Zelda game. What about Rabbids? They're third party if I recall but they have an extensive history with Nintendo (including multiple exclusives, like Rabbid's Kingdom Battle).
As a personnal favorite, Astral Chain should deserve a rep, but I don't know where Nintendo stands with Platinum. Otherwise I've been a fan of having Waluigi in there for eons, despite the bad rep he's gotten through those aggressive campaigns. I mean, he's the missing part in the quatuor: Mario and Luigi, Wario and... nobody yet. Even Peach has Daisy. And despite not having his own games, he is present in quite a lot of others. I think he's a good candidate for the 6th spot on the season pass, if they want to finish with a punch while also not giving in to aggressive campaigning.
Not sure any of the others from the original post's picture have a shot. I don't think they're popular enough or relevant enough. As much as I like Golden Sun, it's a thing of the past and never reached Banjo-level popularity. And instead of the other side-characters, I think Nintendo would go for third-party for the novelty and attract attention through those.