I think the new companies that offer characters with the strongest global prominence are the likeliest to join.
Of course most of the best options in that regard are already covered by the companies already present (including ABK under MS). I suppose options like Koei-Tecmo (w/ Ryu Hayabusa, even though Ninja Gaiden leans west) and Level-5 (w/ Layton, assuming the comeback sticks) would be fairly strong choices from that pool. Others like ASW (w/ probably Sol from GG) and Spike Chunsoft (w/ Monokuma) would be plausible too. And then there's always Travis, but as to what state his series/popularity will be in down the road, who knows.
Choices like Rayman/Ubisoft seem obvious from our vantage, but he really has much less of a foothold in Japan. Not that he couldn't happen, it's just... a very lopsided choice, which hurts his odds. And there's Assassin's Creed, which I suspect is bigger there but... Rayman at least has demand.
Nihon Falcom... may put out a lot of games, and have done so for a long time, but they don't have any characters nearly as prolific generally as third-parties usually offer. I just think other companies have IPs which align more with what Smash goes for.
Of course there's western companies like Take-Two, Embracer, Valve, etc. Most of those series lean fairly western, and lack specific Smash demand, but you never know who might show up as a dark horse candidate. There are certainly big series within those companies. Though if the series barely even exists in Japan, as in, the games don't get released, I wouldn't hold my breath. Oh, and suppose there's also Epic/Fortnite.
I also don't think we can overlook China, particularly in terms of Genshin Impact and (via Tencent) League of Legends. Both of those series are huge all over the globe. Neither has had their main game on a Nintendo system (though both are allegedly still coming), which would, if nothing else, remove some contention around their inclusion, but we'll see how things look down the line.
Plus there's indies. I think next game we'll get at least one as playable. I would, again, look to those with successful global standing as likely options. That's a shorter list, considering many indies skew one way or another. I think most of the likeliest choices got some sort of representation in Ultimate.
Basically I'm of the mind the characters with strong global appeal will remain those who are chosen, and exceptions will exist but be fairly scant and probably somewhat exceptional in their standing, whether in size or demand, to offset the imbalance.