The Roy Effect
Chance: 90%
I think it's highly likely that we get another character from a recent game, which, given we're looking at 2021 releases for the four remaining characters, would probably mean that the game hasn't been released yet. We have a very strong precedent for this, with many Pokemon and Fire Emblem characters being decided upon before their respective debuts. We even have Sakurai on record for both Corrin and Byleth saying that their inclusions stemmed from the desire to give players a character from a game they're playing. Pokemon and Fire Emblem released their latest entries in 2019 though, with one getting a fighter and the other a disqualifying Spirit Event, so it would seem that another franchise has to pick up the slack this time around. Right now it's looking good for Zelda, which is getting Age of Calamity next week and BotW2 soon. A character from that setting would go very well with the increased focus on the series, especially as next year will be its 30th anniversary. However there's bound to be more big Nintendo titles for next year, so it's not a win for Zelda yet.
Even without a fighter, we could easily get content from a new game, say, as a Mii Costume. Or maybe an old character comes with a stage from a recent game. Or it could even be an element from their moveset, like Luigi referencing Luigi's Mansion 3 and Joker using the grappling hook from Persona 5 Royal.
Xenoblade Chronicles DE: idk what happens in Future Connected, maybe something from it could get a Mii Costume? Or if it has new music, it could come with Rex.
Jump Rope Challenge: that's right, the surprise hit of 2020. Idk, something could come from this, maybe.
Deadly Premonition 2: Nintendo put a lot of advertising behind this cult sequel to a cult game. Could amount to something.
Bloodstained Curse of the Moon 2: the only way this gets representation is if Miriam gets in as a fighter, and even then her content would probably be centered around Ritual of the Night. It's a long shot.
Paper Mario Origami King: it already got a Spirit Event so it's pretty much dead. If Paper Mario became playable, I think the 1000-fold arms would be a near guarantee for his moveset, but alas.
Fall Guys: one of the year's big viral games, I guess they could make for Mii Costumes but honestly I think it's Spirit Event or nothing for these two
Battletoads 2020: now technically this one isn't on the Switch... yet. But with Microsoft's tendency of porting their platformers on Switch and Battletoads' history with Nintendo, I'd say the 'Toads have a pretty easy road to Mii Costume status.
Hades: I don't see this one getting more than a Spirit Event, but even if it does it was on Early Access for 2 years before its final release recently. I guess if the content added wasn't in Hades until its final release it would count, but I don't know much about Hades so I'll leave it at that
Spelunky 2: it isn't on Switch but Spelunky has some pedigree and it got great reviews. I could see this getting a Mii Costume, this is the current indie I'd watch (I'd like to say Carrion but I don't think it made much of a splash and it'd be hard to make a costume from it)
Super Mario 3D All-Stars: I won't put it past Nintendo to promote this game through Smash, but even beyond how debatable this is as content from new game (it's just 3 ports), I can't see anything but a Spirit Event for this one.
Kirby Fighters 2: potentially as part of Bandana Dee's content, otherwise this is prime Spirit Event fodder.
Genshin Impact: Wikipedia says this is due to release on the Switch at some point. I doubt Nintendo would want to give the spotlight to the game that blatantly plagiarized them but it is kinda popular so who knows?
Super Mario Bros. 35: I could actually see a standalone stage based on this, kind of like the Mario Maker stage for Sm4sh. But Tetris 99 just got a Spirit Event, so maybe not.
Crash Bandicoot 4: Crash could come with any number of things from this latest entry; hell, he could use his design from this. His Spirits and music are also worth looking out for.
Watch Dogs Legion/Assassin's Creed Valhalla: obviously these two aren't going to be on Switch anytime soon unless the cloud games really sell, and Watch Dogs especially has little chance to get anything into Smash, even tangentially. But a hypothetical Assassin's Creed character could come with Valhalla music, or even an Eivor Spirit, and I could see either of these games getting a Mii Costume.
Pikmin 3 Deluxe: I don't think there's anything that would be specifically from Deluxe instead of regular old Pikmin 3 so this is a firm no
Call of Duty Black Ops Cold War: terrible name by the way. Yeah, we're getting into odd territory right now, but we're rating Alex Mason in a couple of days. I honestly could see a Call of Duty fighter in Smash, and with one of their two most illustrious subseries getting a new game they might as well give it content.
Kingdom Hearts Melody of Memory: while it's just a rhythm game, and therefore has no new music, you can count on this one doing the trademark KH move and feature a ton of new lore and characters and plot and whatever. So look out for Sora's pack as well
Hyrule Warriors Age of Calamity: this one could get a fighter, so anything below that is also fair game
Immortal Fenyx Rising: this game screams Mii Costume to me
Puyo Puyo Tetris 2: even if I don't think Arle is happening, if she did, I think she'd probably take music and maybe even Spirit artwork from this game.
Cyberpunk 2077: it's the biggest game of the year that isn't a console exclusive. It's a longshot but I kinda had to mention it. With Witcher 3 getting ported to the Switch, this one could follow suit, and maybe they'd have a Keanu Reeves Mii Costume or something. Obviously things get easier for this if Geralt (or, hell, maybe even V) becomes playable
Beyond 2020, I'll just name games that could get something: Hitman 3, Prince of Persia Sands of Time Remake, Disgaea 6, Little Nightmares 2, Mario 3D World Furry Bowser Edition, Bravely Default II, Story of Seasons Pioneers of Olive Town, Balan Wonderworld, Monster Hunter Rise, Guilty Gear Strive, NieR Replicant remake, Rune Factory 5, Alex Kidd in Miracle World remake, Axiom Verge 2, Far Cry 6, Halo Infinite, No More Heroes III, Resident Evil Village, that new League of Legends game, Shin Megami Tensei V, Bayonetta 3, Detective Pikachu 2, Final Fantasy XVI, Famicom Detective Club remake, Metroid Prime 4, New Pokemon Snap, and obviously, Breath of the Wild 2. Many of these games I could see getting fighters, others I'd say are just possible Mii Costumes or music inclusions. But I think I've already yapped enough. I just want to justify giving such a high score, as the amount of options is enormous.
I'm going to abstain for want because this could be either great or terrible, and either monumental or meaningless. I'm against including stuff from games when you don't know how they're gonna turn out, at least in principle, but then again, if they had chosen Spring Man for the base roster or Officer Howard for the first Pass you would have seen no complaints from me.
Noms: Cooking Mama x10
Coco as an alt prediction: 28.33%