My guesses for this last spot, if I had to attempt to narrow it down, fall into one of three categories:
a) a Nintendo character
I feel like this goes without saying, but it seems like there's a pretty big chance that the pass will end off with a Nintendo character. I don't think it would be used to promote any Switch games (there haven't been a ton of huge first party releases in the past few months, and there doesn't seem to be many scheduled between now and September, when I assume the next Smash announcement is), so unless we get something announced out of the blue, like a 3D Donkey Kong platformer or another Fire Emblem game, my guesses would be:
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Dixie Kong - she feels like one of those "why isn't she in Smash yet?" cases. Between Tropical Freeze, being one of the player characters in DKC2 (generally seen as the best in the series), and the headliner of DKC3, she seems like the last big Donkey Kong character, and one of the last big Nintendo characters, left to add.
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Cinderace - Pokemon always seems like a possibility; they tend to lean towards the starters, and I feel like Rillaboom would be seen as overlapping with Donkey Kong.
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Bandanna Waddle Dee - this seems like an outside chance, given that when it comes to Kirby, Sakurai seems to prefer focusing on the four games that he directed (Dream Land, Adventure, Super Star, and Air Ride, along with remakes of the first three). However, he's one of the most popular first party picks.
Maybe
Elma if we get a port of Xenoblade Chronicles X announced - they've ported basically every other Wii U game to the Switch - but since we already got a Xenoblade character earlier in the pass, I doubt it. Porky could also happen if we get a Mother 3 port, Mother/Earthbound collection, or Earthbound remake, if they wanted to tie into any of those, but I sincerely doubt that any of those are happening. I realize that having a game on the Switch doesn't mean that a character will get into Smash, but between Byleth, Min Min, and Pyra/Mythra, it seems to help.
b) a new third party company
So far, all of the third party characters in Fighters Pass 2 have already been involved in Smash, in contrast to Smash 4's DLC (which added Square-Enix) and Fighters Pass 1 (which added Microsoft and SNK). I could see them wanting to branch out and use this opportunity to work with other companies. The four that stand out to me are:
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Activision: they have been pushing
Crash Bandicoot really hard lately, and while they don't have any involvement in Smash at the moment, they've had no problem with crossing their franchises over with Nintendo's through things like Skylanders and the Switch version of Diablo 3.
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Ubisoft: they've usually been close to Nintendo, and between the two Mario and Rabbids games and Star Fox appearing in Starlink, that seems particularly true now. If I had to guess which Ubisoft would make it in, I'd go with
Rayman, though I could see
Ezio from Assassin's Creed happening - Smash DLC tends to focus on humanoid characters, presumably because it makes development easier.
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Arc System Works: they've already got spirits in Smash, and they've gotten a lot more mainstream with games like Dragon Ball FighterZ. While I feel like they're less likely since we just got Kazuya and I doubt they'd do two fighting game characters back-to-back, they've done two RPG characters back-to-back several times, so it's still possible. I'd assume they'd go with
Sol Badguy - while Ragna the Bloodedge from BlazBlue could work, Arc System Works owns both characters, and I think they'd go with the original rather than a character who's based on him. While they could go with Billy Lee from Double Dragon, they're best known for their fighting games, so I think they'd pick that to represent the company. (it would be nice to get a beat-em-up character in Smash, but I'd think that Kunio-Kun would have a more interesting art style)
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Koei-Tecmo: if they're going to add a Koei-Tecmo character, it seems like it's going to be
Ryu Hayabusa. Considering that Mai Shiranui couldn't even get a spirit in Smash, let alone a background cameo, I don't see Kasumi from Dead or Alive making it in. As for Dynasty Warriors, I'm not sure if they'd add a character that's based on a historical figure as a playable character - they've added some characters based on real people as assist trophies (Dr. Kawashima and Dr. Wright), but that doesn't seem like the same thing.
c) a Capcom character
With the addition of Kazuya, Capcom's the only third party company involved in earlier Smash games that hasn't gotten a new unique playable character in Ultimate. (Konami got Simon and Richter; Sega got Joker; Namco got Kazuya; Square-Enix got Hero and Sephiroth) They got Ken, but considering what we know about the development of clones/echo fighters (in Melee, even if they didn't include all six of the clones, that still wouldn't give them enough time for one unique character), that's nowhere near the same amount of work. Out of these three broad categories, a Capcom character seems like the least likely, but there are still plenty of options:
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Phoenix Wright: he's probably been requested for the longest, and while his playstyle would be unorthodox (and they'd have to go with something completely different from UMvC3 - I don't think that playstyle would work with the larger stages and expanded mobility that Smash allows for), I could see that being seen as an interesting challenge for the development team, similar to Piranha Plant
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Dante: he's a popular pick, Devil May Cry is in something of a renaissance, and he's got so many moves that I've written four different movesets for him and have only barely scratched the surface of what he can do.
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Amaterasu: I could see this happening as a wildcard pick - Okami's gotten more popular over the years (the HD version being widely available definitely helps with that), it's a very different artstyle from what we're used to in Smash (allowing for some gorgeous stages and tons of references to Japanese culture), and as a Clover/Platinum game, there's no shortage of material to work with.
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Morrigan Aensland: she's Capcom's queen of crossovers, being up there with Akuma and Ryu in terms of making crossover appearances (she's arguably more famous for those than for her original games). As a fighting game character, it wouldn't be a stretch to include her in Smash, and it could work as a way to raise interest in her franchise, which seems to be something that Sakurai likes doing.
There are a ton of other Capcom characters who could be included, though - Mike Haggar, Viewtiful Joe, Nemesis or Claire Redfield from Resident Evil... I could go on and on.
I realize that writing down twelve or thirteen different characters for the single last spot is probably excessive (it's casting a wide net, and even then, I'll probably wind up being completely wrong and it turns out to be a character from a game that I've never heard of, or one that I ruled out for some reason), but I figured I should put my thoughts down.
