Fire Emblem won't be at top of the pile right now. There isn't a new game or a new remake coming out yet, so they'll be digging into perhaps a popular character like Lyn, who's an Assist already. As much as I'd like to play as Lyn, that's really pushing it.
I will now proceed to ramble my thoughts about other first party picks...
Pokémon isn't as out of the running as I'd like it to be. The biggest argument against another right now is that we've already gotten a Pokémon rep in Ultimate, and we don't seem to see a trend of double dipping into series that got a newcomer in Ultimate. Meaning, we got Incineroar, and Byleth, so that's it from those series. The only exception is echo fighters, since we did get Chrom and Daisy, but I think they can be discounted due to how much easier they are to add.
Sword and Shield also got a spirit event in Pass 1, which is usually a bad sign for reps. But in Pokémon's case I can't see that being a prohibiting factor.
Mario got Piranha Plant as a new character. This could contribute to us not seeing Waluigi or Paper Mario as the last challenger, unless they just...decide to.
Zelda has it weird. You coulrd argue that Champion Link is the new Zelda rep in ultimate, he just took the place of Twilight Princess Link. He does have some changes to his moveset to suggest he's not just a carry over. But it does kinda leave the idea of another modern Zelda rep open, i.e. something from the BotW series of games. I think it's unlikely though, since I don't see them picking one of the four champions. There's still AoC fighters like Impa, but we're getting pretty recent for those.
Then all the other Zelda characters you'd expect are assist trophies.
Ring Fit and Astral Chain got very recent spirirt events, which are pretty damning to their chances.
If all the above is true, and we assume for a moment that Assist Trophies are not on the table, then it really narrows down who it could be. We're left with side characters of existing series that weren't repped in Ultimate (BWD, Porky) or characters from new series that aren't really considered that big for Nintendo and are overwhelmingly handheld or Japan only (Chorus kids, Andy, Ayumi).
I don't really consider any of those likely for something Nintendo is picking. BWD certainly has the most community support, but there's many reasons why I wouldn't consider him likely. Andy would awesome since AW is coming back, my problem with him is figuring out a moveset.
Now, I may be pulling these rules out of my ass but my thought process is that it's far more likely we're going to see a rule or pattern get broken before we get something that ticks all the boxes just because it's not already in Ultimate in some way. Not that it means much in the end.