Making predictions for 3rd party characters is next to impossible. I don't think there's any big obvious fan request left, like Sonic and Megaman back then (or Banjo and Sora later on), and Ultimate broke a lot of barriers on who can or can't get in.
Even so, I'll throw my two cents.
Konami: the next character in line has to be Bomberman, and I'd argue he's one of the most likely 3rd party newcomers in general. The series has been around since forever and also seems to be in as decent a shape as a multiplayer-focused maze puzzle game can be in 2024, plus Bomberman himself is both iconic and closely associated to Nintendo. To cap it all off, Bomberman received some kind of special attention in Ultimate, for example with the Mii Costume coming in different colors. I think he's also an easy character to make fit into the game. From Konami I think it's either him or Alucard, other series are either a little too obscure (like Goemon or Twinbee) or not as easy to translate (Silent Hill). Bomberman and Alucard are just relatively easy slam dunks imo.
Sega: no idea here. We could get a second Sonic character, of which Tails, Knuckles, Shadow and Eggman are all excellent choices. Personally I'd like to get Knuckles the most. Or they could put in some new series. Yakuza? Puyo Puyo? Maybe Virtua Fighter... Personally I'd like Ristar, his game is fun, but I know he has 0 chances lol. Ecco is also cool, but in addition to being too obscure he also just straight up can't work. Oh and a Streets of Rage character would also rock. Sega has a ton of cool characters, but I feel like most of the classic ones aren't that mainstream.
Capcom: if I'd have to bet on a 3rd party character in general getting into the next Smash, it's Monster Hunter. The series has already received a lot of attention in Ultimate and it's easy to work with. Personally. I'd love somebody from Resident Evil, Leon has always been one of my most wanted characters, but I have no clue how to make him work. The fact that the series only got spirits in Ultimate doesn't make me very confident in its chances. Then there's Dante, who just feels like a good fit. He's iconic and he has a ton of moves. Chun Li also feels likely, but I'm not sure if it's just because we're now thinking that most 3rd party series are going to pull a Sephiroth. She's definitely one of the few who have 100% earned it, though. Phoenix Wright I don't think it's that likely, the series is not as mainstream as other stuff and Phoenix isn't as natural of a fit into an action game -- not impossible though.
Namco: personally I just want a character from Dark Souls / Elden Ring. The series is omega top tier in the Valhalla of videogames, and they could make the Chosen Undead (or whoever) into a very unique character if they let you, for example, switch weapons on the fly, altering not only your range and strenght but also your weight and movement speed, reflecting the heavily custmoizable nature of the games. Somehow I don't feel like Dark Souls is that likely, though. Tales is another possibility. Or they could add another Tekken character if they manage to make them work? Jin and Heihachi are definitely big deals. Aside from these three series, I don't know... I'm not a big expert on Namco.
Square: Geno's stocks are at all-time high at a very strategic moment. Maybe it's finally time for him to get in. Then there's 2B, who's kinda likely, but I think her moment was in Ultimate as a Joker-style pick. Another character from Final Fantasy is a possibility, but as much as I'd like for them to branch out of FF7 and add, I don't know, Vivi with a Lindblum stage and all that sweet FF9 music, I know the next character is just going to be Tifa lol. Crono is also worth mentioning because everyone loves Chrono Trigger, but he's not very likely. Meh, anything could happen with Square.
Microsoft: impossible to predict what's going to happen here, or if they'll even want to provide more characters. Banjo and Steve were very special cases, as a former Nintendo character and as the protagonist of Minecraft. You could narrow it down to Master Chief / Crash / Doom Slayer as the most requested, but they own so many IPs now... Personally I think Doom Slayer would be the best choice, but I'm biased - I love Doom. Halo kinda feels like it could still be a step too far, even with how much Nintendo and MS cooperate, and Crash... I don't know. There's nothing wrong with Crash, he just gives me unlucky vibes for some reason.
Ubisoft: there's a non-0 chance they pull a Megaman/Ryu or a Banjo/Steve, with Rayman and Ezio (or any character from Assassin's Creed). First the more fan-servicey, more closely associated to Nintendo character, and then the more mainstream one. I see nothing wrong with Rayman as a choice, he does feel a little B tier (no offense to Rayman fans), but he's still very well-known and his funny abilities are enough to endear him to audiences not familiar with the character. Plus Rayman music is great. He's just easy to envision, which at least for base game I think is a plus.
SNK: anything could happen, but personally, since Terry already covers the fighting game side, I'd love them to put in a Metal Slug character. Maybe all four in a single slot, Hero-style. I don't think the final moveset would be too unique, as a lot of stuff you do in Metal Slug is kinda already covered by other characters already in Smash, but eh it would be cool.
Indie: ??? I think Undertale has the best chances of getting a character, with Cuphead and Shovel Knight not too far behind. Hollow Knight also feels possible. For Undertale I'd like to get Frisk more than Sans. He's harder to translate into Smash but I think he could be more unique.
Other: there's Ryu from Ninja Gaiden, he was pretty requested during Ultimate, I think he could happen as a classic NES pick kinda like Simon, I don't think he has much competition in this department, unless you want to count Bomberman. There's also other big publishers which I don't think are all that likely to get a character, like Rockstar or EA.
My pick? Crazy Dave from Plants VS Zombies, make him plant all different kinds of plants around the stage to get stage control in the weirdest ways. I think he'd be super fun, but I also think he'll never happen.
Just free-flowing thoughts. I won't make predictions because it's impossible