Mine:
: It's happening. It might be Geno, it might be Paper Mario, it might be Captain Toad, but I'm more certain of this happening than any other franchise still.
: Again, Pokemon, like Mario, usually get multiple characters as they're the BIG TWO of Nintendo(Sorry Zelda). Even with the return of Squirtle, Ivysaur and Pichu(as well as Leaf joining Red as an alt for Pokemon Trainer) I could easily see another new mon join the squad.
: The outcry has surely reached Sakurai's ears with cries for the antagonist, the tritagonist and even Cranky and Funky getting support to some extent. Only reason I'm less certain than the other two is DK has...basically gotten barely anything over the years, so maybe Sakurai just doesn't wanna. No Rambi assist trophy even which I find stunning.
: Isabelle. Unlike the above, this entirely is based on Isabelle's likelihood rather than multiple other characters. She's more popular than the majority of other characters combined with only her pacifism arguably stopping her. Said pacifism which was a non issue in Mario Kart 8- she's happily blowing people up with bombs and shells, so it may not be as big a deal as people make out.
: Dee or bust IMO. We could get a surprise like Marx or my personal favourite Gooey(highly unlikely despite him being the Luigi to Kirby briefly) but Dee is a much bigger star. Only putting this low because Sakurai may feel his trio of protagonists is enough as Dee got his big roles after Sakurai stopped being the head of Kirby and frankly Mario and Pokemon generally get better priority, Isabelle is insanely popular and DK has been desperate for any kind of content forever.
: Elma, Cross(who often gets looked over but is technically possible still), Rex and Pyra possibly if sketches were a thing at the time. Definite possibility, just the series is still fairly niche compared to the above...not that it's stopped Sakurai in the past, but I think newcomers from the above series would draw a bigger crowd overall.
: Ok, I know. Fire Emblem. But it's become a much bigger household name since Sm4sh and Heroes has been the biggest SMASH(heh) hit for the Mobile market. They missed a trick with making Lyn an assist. Girl's incredibly popular, but there's many others to choose from.
: Echo or bust I feel. Impa is likely as has gained a bit of a fandom since Sm4sh, but I don't expect a unique newcomer from the series after Link, Zelda and Ganondorf all got moveset overhauls and Young Link was brought back too.
: Possibly Dark Samus as an echo or semi clone but I think Sakurai may think Metroid fans are satisfied with their big purple boy now and I can't see him added Anthony Higgins(mores the pity) or Sylux just yet.