Banjo, Steve, Sans, Frisk and Crash are the ones (Western third parties popular in Japan) that stand out off the top of my head as having a chance.
This fanpoll places four of them in the top 18, and Frisk (the odd one out) is confirmed to make top 20. I don't know how long any of them have had Smash support (if I had to guess, Banjo had it the longest, then Crash, then Steve, then Sans, and Frisk shortly after), and more importantly while I know that fanpolls inherently can't properly represent the fanbase as a whole since they have such a small void, Banjo's the only one I'd question still being popular among the general public.
(As an aside, Arle's the highest ranking new series character for SEGA in the west, and in Japan she just is the highest ranking SEGA character. She BARELY didn't make the poll's front in Japan. If we get a second pass, or if third party newcomers are still a thing next game, I'd dare say she really should be on all our radars until further notice. I'll not bring up this one because everybody will just spam Ren Amamiya's presence, even though a bunch of people really believe that we could get both Banjo and Steve.)
Here's a warning: if we do get Banjo,
it's likely he'll be the only major Smash Fandom pick. Smash Fandom is defined as people like us, who are active online and big requestors of the characters. But more likely than not, with him they'll have that base totally covered. I would expect characters who are more niche here but bigger in other parts of the gaming world to get in, mostly because they'll tap into a totally new audience. Don't let this be K. Rool round 2, where everybody and their (grandparents think that we'll get everything and end up being disappointed when it doesn't happen.)