Waluigi: He's an extremely popular character in general and I'm sure the negative reaction to his Assist Trophy confirmation reached Nintendo. I can see him being the Ridley of the next game, as in one of the first newcomers announced to get people hyped for the game. His moveset could be used as a way to showcase all of the Mario spinoffs like Mario Party, Mario Kart and all of the sport games.
Dixie Kong: Dixie has had bad luck getting into Smash Bros. She was planned to be paired with Diddy back in Brawl, but that didn't happen for one reason or another. In Ultimate, the Donkey Kong series finally got a new character, but unfortunately for Dixie, it came in the form of King K. Rool. Now, she's put in the position as the most logical Donkey Kong newcomer. It just makes sense to me for her to finally be added in the next game. As for her moveset, at worse she could be a semi-clone similar to Isabelle, but I think she could be made to be more unique than that.
Sidon: The Zelda Series hasn't had a newcomer since Brawl and it feels like it's about time for it to get one. While I'd like to see Skull Kid or Midna, I feel like a BotW/TotK character is what Nintendo would go for. Out of those characters, Sidon has been the most consistently popular out of all them. As a Zora, he'd stand out among all the Zelda characters, too. He'd also be Smash's first trident user and I think that's pretty neat.
Bandana Waddle Dee: Like Zelda, the Kirby series hasn't gotten a new character since Brawl. It feels even more egregious somehow to me. Maybe it's because unlike Zelda, Kirby hasn't gotten stages that represent the newer games in the series? Whatever the reason, I feel like it's time for the Kirby series to get a new character and Bandana Waddle Dee has essentially become the fourth main character after the trio already in Smash. He'd be the second spear user after Byleth, too.
Isaac: Fun fact: Golden Sun is now older than the Ice Climbers when they got into Melee. While this qualifies Isaac as a retro rep, that's not my reasoning for his inclusion. Golden Sun somehow got 8 spirits in Ultimate, the most out of all non-playable series in Ultimate. It makes me think that Nintendo knows people have fond memories of Golden Sun and I truly believe that Isaac would have gotten in Ultimate if the base roster had more newcomers. While Isaac does wield a sword, I think he'd easily stand out compared to other sword users thanks to his use of Psynergy. There's no character in Smash who uses Earth magic and Isaac could fill that niche, alongside some other non-Earth based Psynergy like Move. Felix can come along as an echo, too.
Octoling: I haven't played Splatoon yet and I still know that it has become a massive franchise, so much so that only having one character in the next Smash would seem strange to me. From what I understand, Inkling's moveset is currently only based on weapons from Splatoon 1. Keeping that in mind, weapons from Splatoon 2 and 3 could be used to give the Octolings a unique moveset.
Meowscarada: This could be any Pokemon from the latest Pokemon generation at the time the roster is decided, I just used Meowscarada as something of a placeholder. Every Smash Bros has at least one Pokemon newcomer and the next one won't be an exception. I do think Meowscarada makes a lot of sense as a Gen IX rep, though. The way it's been pushed reminds me of the push Incineroar got back in their debut generation. Liko from the Pokemon Horizons anime even has Sprigatito as her starter.
Takamaru: I like NES retro characters and I like Takamaru the most out of all possible retro reps. Only Star Man from Pro Wrestling comes close. Takamaru was considered before, but didn't make it because of his game's obscurity. This was mostly due to the lack of an international release, but it has now released outside of Japan and is even part of the NES games offered by the Nintendo Switch Online service, improving Takamaru's chances a bit. He's still a little obscure, but I just like the idea of a samurai character in Smash.
Ayumi: Ayumi is a very interesting case. As many people know, she was once briefly considered as a playable character for Melee, but was decided against due to lack of relevance outside of Japan. More than 20 years later and the Famicom Detective Club games have not only gotten localized remakes, but an actual sequel. Ayumi is now relevant and her inclusion could finally get Smash Bros a chance to do a Visual Novel character.
Legionis: Astral Chain was a fairly succesful game and Nintendo bought the rights to it from Platinum. This makes me think that Nintendo probably plans on turning it into a series and if that's the case, getting a Smash Bros character makes too much sense. The moveset for the Legionis is oozing with potential, too. A puppet fighter that can attack from afar with a Stand is such a cool concept and is what I thought we'd get when Joker was announced.
Ring Fit Trainee: Never played Ring Fit Adventure, but I know it sold extremely well. As one of the biggest successes on the Switch, it'd make sense for it to get playable representation in the next Smash Bros. From the gameplay I've seen, the Ring Fit Trainee could probably have a fun moveset that differs from the Wii Fit Trainer.
Geno: Yeah, I think he's happening next game. I'm not the biggest Super Mario RPG fan, preferring the other Mario RPGs, but the remake has sold very well, even better than the original release, making me think that it's finally Geno's time. In my eyes, his lack of relevance has always been one of his biggest obstacles, alongside competition from other Square Enix properties. He's now relevant again and Square's biggest franchises are already in the game. Nier Automata and Chrono Trigger are the Square Enix series that I feel are missing, but they seem like something that'd be reserved for DLC, if they even happen. In contrast to those two, Geno could easily be a base game character, only needing a couple of songs from SMRPG added to older Mario stages. His moveset could probably have a timed hit mechanic, dealing more damage if timed correctly.
Shadow: The Sonic franchise is overdue for a second rep and Shadow has been popping off lately, what with the Sonic 3 movie and Sonic x Shadow Generations. He wouldn't be a clone at all, either. That new game has given him some really cool moves, but even without them, he could be very different than Sonic.
Ryu Hayabusa: Koei Tecmo and Nintendo have a very good relationship that hasn't resulted in a character yet and I feel it's time for them to get in. Hayabusa could play a similar role to Simon Belmont as a sort of retro NES third party character. His moveset potential is huge, too. The only thing I'm absolutely sure is that the Izuna drop must be a command grab so he's able to use it midair.