Regarding first-parties, I don't have the slightest idea who Nintendo would want to add.
- Captain Toad or Paper Mario, I guess. Not sure how likely they really are, but if Nintendo is calling the shots, I guess any prominent Mario character would be expected to sell well.
- Suprisingly, this is the hardest franchise for me to pinpoint a frontrunner. Even though most people assume Dixie, I could see Nintendo pushing for Funky or even Cranky as the next DK rep. All three are pretty iconic in their own rights.
- By the process of elimination, the only realistic Zelda characters left who haven't already been disconfirmed by ATs, stage hazards, and FSs are Impa, Toon Zelda/Tetra, and the Champions. Between those three, the Champs are the only ones who I could actually see Nintendo wanting to promote on the Switch. BotW did phenomenally well, and it continues to sell copies up to this day.
- Sylux, but I'm not sure if MP4 would've been taken into consideration. Depends on how much time and money Nintendo has invested in it.
- I'd say Bandana Dee, but honestly, it's hard picturing Nintendo choose BWD out of everyone else. He isn't the best promotional character.
- Gen 8 rep, but same thing as MP4. Depends on how much planning ahead of time was done.
- Three Houses rep. Same as MP4 and Gen 8, though I think FE would be a lower priority than those two.
- Elma. mainly because it seemed like Sakurai outright deconfirmed Rex. Can't think of a reason why Nintendo would want to promote Elma though, especially since her game was on the Wii U.