My series by series ramblings on who is most likely and/or I'd like to see more:
Toad: his stocks are at an all-time high. He keeps showing up in mainline Mario games, he got his own game (which got rereleased twice!), he's the funny sidekick in a kids' movie which often guarantees popularity, and he's an uber iconic character in general. There's lots and lots of source material to draw from to make him a fighter. I believe the only thing that can stop him this time is if Sakurai really is dead-set against it. We don't seem him as likely because he got skipped over in the previous games, but in a vacuum I'd say he's a no-brainer.
Geno: another character whose stocks are at an all-time high. He's always been heavily requested, but now he's finally relevant and the Mario RPG remake did very well in both sales and reception. With Square being more collaborative lately, and having most of their major characters already in Smash, it feels like the time could be right. I give him a slight edge over Toad because Sakurai clearly likes the guy.
Waluigi: to be honest, I don't see him happening. Toad is more popular with casual fans, and Geno has a longer history of being requested for Smash; I'd also say that both are easier to make a moveset for that would make them unique. Waluigi got that uproar when he was revealed as an AT during Ultimate's first presentation, but that's pretty much it. The fact that he's still relegated to spin-offs doesn't help his case. Don't get me wrong, the dude is funny and I'd like him to be playable, it just feels like there hasn't been any major shake-up that really raised his stocks unlike what happened with Geno and Toad.
Paper Mario: I feel like he's more likely than people give him credit for? I think we can all agree that, if the series featured original characters, it would have had playable representation since Brawl. It can still happen, and Paper Mario has a lot to work with to make a moveset, being able to draw stuff from at least 6 games. He also has multigenerational appeal, having appeared on pretty much every Nintendo console from the N64 onwards, and the series is still going strong... Could happen.
My pick: Toad
My prediction: Geno
Dixie Kong: what's there to say that hasn't been said already? Dixie has appeared in a ton of DK games, she co-stars in DKC2 which is probably the most beloved entry still, headlines DKC3, got brought back for Tropical Freeze to huge acclaim, has always been a fan-favorite, gets used in Mario stuff, appears in movies, Lego sets and theme parks. This character has proved herself time and time again, and to top it all off she possesses unique abilities that aren't represented by any other DK character in Smash. However, DKC hasn't gotten a new game in forever, and Smash has always been very reluctant towards adding content from this series for some reason. She doesn't feel particularly likely, all things considered.
Funky Kong: he'd be a perfect echo fighter for DK. Make him do the same attacks but with a different flair, in a Samus/Dark Samus-like dynamic. He'd be a show-off tryhard but still play the same. Maybe make him spin on his surfing board during the Up B. It feels like he's popular enough to get in this way.
Cranky Kong: could still happen, but he's a distant third compared to the other two. A comedic moveset themed after senility and 4th wall breaking would be very funny, but it's a matter of Sakurai getting that idea and developing a strong enough affection for it to make Cranky into a fighter. In terms of fan requests and ease of development, Dixie and Funky both trump him.
My pick: Dixie Kong (+ Funky as an Echo)
My prediction: none
Rauru: I'm fully convinced this character is extremely likely. Newer Smash movesets have focused a lot on bringing in not just the character, but the entire mechanics of their game of origin - and you need a full moveset to do TotK's mechanics justice. It can't be done by just changing a couple of Link's moves and calling it a day (plus I believe Link should stay BotW-themed, it's going to be THE Zelda game going forward). Link's abilities in TotK canonically come from Rauru so he'd be a great fit to represent Ultrahand, Ascend and all the other stuff. With the BotW duology propelling the series to new highs, it just feels right to have a character from it, and Rauru's the best fit.
Skull Kid: if they decide to look back at older characters for a Zelda newcomer, there's still a strong case to be made for Skull Kid. Majora's Mask is stil a very popular game and that's not going to change any time soon. You can build a moveset for him without too much trouble, and he's also been significantly requested for a long time. I'd say that looking at older Zelda games right after BotW/TotK is particularly unlikely though, and I dunno he also kinda hinges on Young Link not getting cut? Would be weird for the main "villain" of Majora's Mask to be playable while the hero is nowhere to be seen.
Midna: you can apply to her most of the points I've made for Skull Kid; I just feel like Skull Kid has a bit more clout in the general gaming sphere.
Ghirahim: he was a front-runner along with Skull Kid and Midna once, but unlike those other two, it feels like his popularity has faded a bit. Extremely unlikely in my opinion.
Impa: people always rave about her, but eh I'll be honest: I kinda don't want for the first original Zelda character since Melee to mostly draw from a spin-off. This is a Hyrule Warriors character, not a Zelda character. In the main series, her design and abilities are all over the place and she's not really super memorable. ...aside from her killer Skyward Sword design. If they find a way to work that into a Sheik echo, I'm on board.
Ganon: it's worth bringing him up but I don't see him getting in as his own character. I think it would be cool if they found a way to implement him in Ganondorf's moveset, though. Like, I dunno, after he reaches 100% damage he transforms into Ganon... Probably a dumb idea, but it would be nice for Ganon to be more involved.
My pick: Skull Kid
My prediction: Rauru
Raven Beak: I think this dude is one of the best final bosses in any Nintendo game, which is high praise. A long, adrenalinic, unique, tough but fair fight with a bombastic finale! And he would work pretty well as a fighter. But he's still a one-off villain and those usually don't get in Smash, and also I don't know, loved Dread and all but personally I'm fine with just Ridley and Dark Samus.
My pick: give Dark Samus some more of her own abilities and call it a day
My prediction: none
Poochy: love the little guy, but let's be real.
The stork: would be very funny, but let's be real.
Shy Guy: this is more like it! Would be a bit of a repeat of Piranha Plant, but these guys are a core part of Yoshi's identity as a series and with all their different occupations there's lots and lots of stuff to draw from to make a moveset. ...but let's be real.
Kamek: the only candidate "from Yoshi" with a tiny chance, Kamek has unique abilities and shows up very often but I'm not sure he's ever really evolved past being mostly a background character. He has a notable role in the Mario movie, which is nice, but in the games... eeh. Can't see it.
My pick: none
My predicion: none
Bandana Dee: Kirby's had, like, 5-6 important games since Brawl (Return to Dream Land, Triple Deluxe, Robobot, Star Allies, Forgotten Land - if I haven't forgotten anything) and I believe Bandana dude plays an important role in all of those as well as in the spin-offs. He's heavily requested, the spear moveset is fun, there's not really any character in Smash who plays like he would (small body + big range). If I have to be perfectly honest he's not particularly awe-inducing as a choice, but objecively there's no reason why he should get skipped over again.
Magolor: a step below Bandana Dee, but he's one of the few Kirby villains who escaped the "appears in only one game" curse and has a suitable body shape to work as a fighter. I don't remember much from his fight in Return to Dream Land but it feels like he can be unique enough. Representing the more cosmic, "dark" side of the Kirby series in a character would also be interesting.
Marx: he's a lot like Magolor, in this case they'd go vintage instead of modern. I think I'd like Marx a little more because it would be cool to see how they make a fighter with no arms move around, and his jester theme is more interesting to me. I also think he's creepier than Magolor, at least from what I remember from the games.
My pick: Marx / Bandana Dee
My prediction: Bandana Dee
Krystal: if this series was ever to get another character, it would be her. The staff abilities in SFA still feel unique enough even after all this time, to be honest. But it's not going to happen.
My pick: none
My prediction: turbo none
I'm super out of the loop with Pokémon. I usually like what they bring to Smash though, so it's going to be a Gen 9 or Gen 10 Pokémon and I'm most likely going to like it. My last games have been Gold and Silver so I'll just talk about the two Pokémon I'd like to see the most.
Gengar: a trickster ghost would be pretty unique and I love his creepy smile. I also actually think he has a shot, really proved himself to be an evergreen character in terms of popularity.
Heracross: he doesn't have a shot. But I think a Bug Pokémon would be very cool, and Heracross has the coolest, most Smash-fitting design, at least out of the ones I know. He could fly around, ram into you with his horn, harden his skin, shoot webs, do some wrestling moves... I'd like him a lot.
My prediction: a Gen 10 Pokémon
Blood Falcon: would make for a pretty cool Echo fighter. Ultimate's Classic Mode routes already use Captain Falcon's Blood Falcon color scheme as the actual Blood Falcon a ton of times anyways, so they could just go all the way and make him an echo. Change his voice, make his animations less heroic and his flames purple (and of course change the Final Smash to use his machine) and he's good to go. I don't see much else happening with F-Zero, even Blood Falcon is a long shot. If he appeared in F-Zero 99 I'd be sligthly more confident about his chances, but that's not the case.
Black Shadow: people want him to inherit Ganondorf's moveset, but I don't think that would work. The sword attacks would be out as Black Shadow doesn't use one, and some of the fully original moves (ftilt, fair, side B) are references to things Ganondorf actually does, so it would be weird to take them away from him and give them to a different character. Also, Black Shadow is still an F-Zero character. I don't think he should be as fast as Captain Falcon, but certainly not one of the slowest characters in the game either.
Pico: just bringing him up for fun, but a violent futuristic ninja turtle assassin with a penchant for guns? That would make for an awesome character who'd retain a lot of that aggressive, precise F-Zero feel. Plus he's been there for the whole series, and for what it's worth, he (or at least his machine) is at the peak of his popularity thanks to F-Zero 99. Samurai Goroh and Dr. Stewart have also been there since the beginning, but I feel like they'd be a little more difficult to make a moveset for. As a big fan of F-Zero I feel like Pico would be the ideal pick. But it's not going to happen.
My pick: Pico
My prediction: none
Porky: Porky would be amazing. I think he would fight in his Earthbound mech since it's smaller and more compact, but he'd have stuff from both it and his spider mech from Mother 3. He rams into you, releases stinky gas, deploys the Porky Bots, shoots lasers... He could also take stuff from some of the robotic enemies in the Mother series, like raising up shields and the likes. Final Smash would be of course summoning the Devil Machine, turning it off and leaving everyone to the mercy of Giygas. Aside from potentially being very unique and bringing in Giygas (who's one of the most iconic things about Earthbound) he also has two advantages: being the vililain for both Ness and Lucas, and already having had an important role in Brawl, which makes him more well-known to general Smash fans compared to pretty much any other Mother character. Maybe if Mother was still going on as a series, and/or if Smash didn't break it big the way it did, he'd have a decent chance. As of now, though, he doesn't really.
Masked Man: I remember he was heavily requested during the Brawl days, but that time has passed, and being Mother 3 only really kills his chances. Realistically, Mother is not going to get another character, and even if it did, it would 100% be Porky. A shame since he's obviously cool.
Ninten: a potential Ness echo? I love Mother 1 but I don't feel much need for this. He's too similar to Ness to even be an echo.
My pick: Porky
My prediction: none
...ended up geeking out way more than I though I would, so for now I'll just leave it at the Smash 64 series.