Waluigi:
The "Ridley" of the next Smash Bros. People have begged and Sakurai is usually pretty good with adding highly requested characters into the following entry, and boy have people been vocal about wanting Waluigi.
Marx:
People have been clamoring for a new Kirby character and Sakurai might heed their cries, but I could see him going an unexpected route. He loves putting content from the Kirby games he directed into Smash, and Marx is arguably the next most remarkable character from those games. There's evidence he has a soft spot for him considering Marx is both a boss and a Mii Fighter hat in Ultimate...
Noah:
Xenoblade is one of Nintendo's highest growing franchises, so adding the protagonist from the latest entry is practically a shoe-in. Not to mention the post launch Xenoblade 3 spirit shows credence to the idea of him in Smash.
Mallow and Geno:
One of those 2-in-1 characters like Pyra/Mythra and Zelda/Sheik in Brawl and Melee. I do think it's high time we get more substantial SMRPG content in Smash in light of the remake, but it would feel kinda weird to have Geno playable without Mallow. Think about it, every single party member would be playable except Mallow! Both are equally important parts of that game and picking one ove the other just wouldn't sit right with me.
A New Pokemon:
Kinda obvious right? I don't think it will be from gen 9 though. I'm pretty sure Sakurai was given early design documents for both Greninja and Incineroar, so it could be the same with this mysterious gen 10 Pokemon. (Let's be honest, it's probably a starter.)
Ring Fit Trainee:
Ring Fit Adventure is Nintendo's most successful new IP of the Switch generation, with at least 15 million sales. Nintendo clearly sees it as a valuable IP, going as far to add Ring Fit as one of the five games represented in Nintendo Alarmo at it's launch, along with heavy hitters like Mario Oddysey, Breath of the Wild and Splatoon 3. They'd fit right at home with other oddball picks like Ice Climbers, Villager, and (obviously) Wii Fit Trainer.
Ayumi Tachibana:
Speaking of oddball picks... Famicom Detective Club saw a surprising revitalization in the past few years, with the remakes of the original two games that actually got localized in the West and the completely new third entry that Nintendo was keen on advertising. Ayumi's the face of that franchise, being it's only representative with the trophy in Melee and spirit in Ultimate. As for how she'd fight.. This is Sakurai we're dealing with here so there's no doubt he'd come up with a moveset that incorporates elements from her series. (Maybe she'll find clues/evidence like Pheonix Wright in UMVC3?)
Bomberman:
Yup. It's him. Bomberman is one of the few third party franchises to be represented as an assist trophy with no playable representative, the others being Monster Hunter, Shovel Knight, and Virtua Fighter. I feel like he's the biggest Japanese gaming mascot to not be playable in Smash yet, being the face of the (now defunct) Hudson Soft. A historically important and legendary franchise in the same vein as Mega Man and Castlevania, he seems like prime base roster material to me.
Kunio:
The hot-headed highschooler from the River City series, known in Japan as Nekketsu High School. This one's more for the Japanese audience, though Kunio does have his fans here in the West, namely for the awesome NES brawler River City Ransom. He's a Famicom icon over there, with dozens of games ranging from sports to beat-em-ups with new games being made to this day. I don't really see him being DLC due to his sorta niche status over in the West, so if he's gonna get in, getting in the base roster is the best shot he's got.
A "Surprise" Character:
It's tradition at this point. We got Mr. Game & Watch, R.O.B., Duck Hunt, Piranha Plant, you know the drill. I always look forward to seeing what unlikely/obscure facet of Nintendo's history Sakurai can conjure up into being a fighter. What will it be this time? Will it be Diskun, the mascot of the Famicom Disk System? A throwback to Nintendo's hanafuda card days? A playable Ultra Hand? The Reckless Wii Remote Safety Guy? Anything's possible with this series.