Since questions always do well, what first party well established series you guys expect to see newcomers next time ? Hard mode: no mention of Mario, Pokemon and Fire Emblem.
Personally I see Xenoblade, Animal Crossing and Splatoon as inevitable , even IF Animal Crossing and Splatoon newcomers end up being Echoes, Isabelles or semi clones. Likely are any sort of Zelda newcomer, and I sort of hope for a DK newcomer because Cranky appears in the Mario Movie, and he'd be a great character to give the original Arcade game a character on terms of his moveset.
I'm just going to post my rambling thoughts in no particular order. (not counting series that are only one game, Wii Fit, or the retro ones)
Splatoon: incredibly likely, though I'm expecting it will be Octoling as an echo rather than a unique character
Xenoblade: I'm not sure how well Xenoblade 3 did, or how well it was received, but I figure it will get something, whether it's base game or DLC
Animal Crossing: on one hand, the success of the Switch game makes it seem likely, but on the other hand, I'm not sure who they'd add. (not familiar enough with the franchise beyond the Gamecube game to know who stands out, if anyone)
Donkey Kong: here's hoping; maybe the push for K. Rool made it clear that people want more Donkey Kong content? (Donkey Kongtent?) Dixie and Cranky seem most likely to me
Zelda: I want it to happen (it's been over two decades since Smash got a new unique Zelda character despite Hyrule Warriors and Age of Calamity making it clear how much potential the series has), but Smash seems content to avoid adding new Zelda characters like the plague
Metroid: I feel like we were lucky to get Ridley and Dark Samus as playable; I have my doubts that there will be more. (maybe Raven Beak, but I doubt it)
Yoshi: I seriously doubt it; Yoshi being considered its own series in Smash feels like an artifact of the N64 days
Wario: as much as I'd love to have Captain Syrup for some Wario Land references, I doubt it
Star Fox, Kirby: there's potential, but I seriously doubt it (Star Fox hasn't gone much further in terms of its timeline, and Sakurai seems content to focus on the Kirby games that he directed)
Earthbound: nothing seems to have changed since the Brawl days for the series
F-Zero: while some characters would be fun to see playable, I don't see it happening
Kid Icarus: maybe? It's been long enough that people likely wouldn't be annoyed by more Kid Icarus content or consider it Sakurai bias or what have you
Pikmin: probably not, though I could see Alph being promoted to an echo fighter
Punch-Out: I could see it happening; maybe Doc Louis or one of the opposing boxers
TL;DR:
Likely: Xenoblade, Splatoon
Maybe: Punch-Out, Zelda, Donkey Kong, Animal Crossing, Pikmin
Unlikely: the rest
(at least in my opinion, though I'm speculating completely blind here)