The most likely candidates right now are:
-K. Rool (seeing how popular demand made Ridley playable he has a decently good chance)
-Rex & Pyra / Elma
-Dixie as a clone
-Gen 8th Pokemon he sees worth or no new pokemon reps
Unexpected curveballs that Sakurai may consider (even as DLC):
-Isaac
-Banjo
-Springman
-Belmont
-It either has to be a new FE16 rep or no rep at all
-Crash
Never gonna happen but are expected for some reason:
-Decidueye (seriously who came up with this)
-Lycanroc (same thing as above)
-Mimikyu (seriously?)
-Bandana Dee
-Toad / Captain Toad
-Rhythm heaven rep
-Impa
-Ashley
-Geno
-Sora
-Medusa
-Shantae
-Shovel Knight
-Sans
-Any champion from botw
-Midna
-Skull kid
-Minecraft Steve (how is this even a thing?)
I agree with King K. Rool, Dixie, and Rex/Pyra.
Gen 8 Pokemon though...? Really? We don't even have the NAMES of those titles yet. They aren't even going to be revealed until next year and the games likely won't even come out until holiday next year. There's no way it'll happen unless it's DLC.
Decidueye/Lycanroc/Mimikyu are all likely Pokemon newcomers. Very popular in their respective games and they all have uniqueness to them. Decidueye is an archer that can fly. He can use Spirit Shackle, Brave Bird, Leaf Blade, and his Final Smash could easily be his unique Z-Move, Sinister Arrow Raid. Lycanroc has 2 different styles they could use. Bipedal Midnight form or his quadrupedal Midday and Dusk forms. He could use various moves like Accelerock, Stone Edge, Crunch, Counter, and again his final smash could be Splintered Stormshards. Mimikyu has a very unique gimmick in Disguise which could be utilized in a very creative way in Smash and could use moves like Shadow Claw, Play Rough, Wood Hammer, Feint Attack, and it also has a unique Z-Move that could be a Final Smash in the form of Let's Snuggle Forever.
Your 'curveballs':
Isaac, doubtful. Wasn't he removed as an Assist in Sm4sh? Why would he make a comeback as a playable character?
Banjo, big miss. Hell no.
Springman, not much of a curveball. Pretty sure everyone is expecting an ARMS rep.
Belmont, despite the 'leak', I'm still saying no. Ridley has been 'leaked' dozens of times(honestly...), Ice Climbers were a given, and anyone can 'guess' every fighter from the previous game would come back. I'm pretty sure most people are sick of third party newcomers at this point as well.
FE16 rep, nope. Maybe as DLC, but not on the main roster.
Crash, yeah, no. I just don't see that being likely.
'Never gonna happen':
Bandana Dee? Eh. Agreed. If they're going to do a Kirby newcomer I'd expect one of the Star Allies characters like Marx.
Toad is already deconfirmed, Captain Toad is still a possibility, but I don't think it's as likely as people think.
Rhythm Heaven rep, ehh... maybe? We know there was definitely one meant to be in Sm4sh from coding, but who knows if they ran out of time or just gave up on it altogether.
Impa, disagree. She could be a very likely Echo Fighter for Sheik.
Ashley, Geno, Sora, Shantae, Sans, Shovel Knight, Steve, all agreed. Big no. Midna is also an assist so she's deconfirmed.
Medusa, disagree. Probably the most likely Kid Icarus newcomer. Not sure if they'd go for another KI newcomer though.
Champions from BotW, slight disagree? If we're to get a new LoZ newcomer, I'd expect it to be one of the BotW champions, probably not Daruk, but the other three I'd say have a decent chance.
Skull Kid, might be wishful thinking here, but if we get a LoZ newcomer that is a villain, he'd be the most likely IMO. Very large fanbase and comes from a very popular LoZ game.