I don't think everyone will return either, but it's far more likely if this Smash becomes a direct port, and thus basically copy pastes a lot of Smash 4's data, and character work. In that case, a character can become cut due to dificulties in development, similary to Ice Climbers, time constraints, or by choice.
This time, I have honestly no idea who will get cut and who not. I'd just state that third party characters are not too likely to all stay in the game. Sonic is pretty much safe I guess, and I think Bayonetta and Cloud are also very likely to stay. Yet the others am not too confident in.
Shulk could also get cut I feel, he strikes me mostly as a 'one time addition' than most others on the roster. He could simply be replaced by Xenoblade's sequel's main character. Sakurai even stated that characters like him don't normally make the roster. I think that's saying something...
Wii Fit Trainer had lukewarm reception, and could also be seen as a less serious addition that was added to Smash by the development team having fun. Is Wii Fit still as popular as before? I honestly doubt it...
Mii's took a lot of development time, and they are easily the least popular characters in the game. Know anyone who mains Brawler, Gunner or Swordfighter? No, neither do I. These characters should be the first to be considered cut if they looking to spend less development time in brining veterans back. They are pretty useless filler characters honestly.
Dark Pit is easily the least popular clone, character wise. Lucina and Dr.Mario are safer, Dark Pit is likely not to return in Kid Icarus... And Kid Icarus itself also isn't very likely to get a new game. Dark Pit was added when Uprising was all new and all that. I do not think that it's justified to keep Dark Pit around if the franchise isn't running active anymore. Pit and Palutena is enough.
Jigglypuff might even be removed, as it was low priority in Brawl as well. It's literally the only current Pokemon on the roster without insane popularity, and isn't promoted at all anymore. I could see her being cut, especially if we get another Pokemon newcomer and the roster becomes too crowded. Easy DLC content too- don't forget that.
To be quite honest, I could see the Zelda roster getting a major overhaul. And thus meaning the end of characters who are not Link or Zelda... Err... Yeah, so I meant their alternative reincarnations Sheik and Toon Link. But even Ganondorf could be cut, as he's not that much present in the series anymore. BotW's DLC might change that, but honestly, the Zelda roster is a mess.
-It has Zelda and Sheik split up, but due to their movesets being all about """""""BALANCING"""""" the other out (as in, making Zelda next to useless and Sheik overpowered and fast so she can easily 'rack up damage') and thus finally resulted in the majority of Smash 4's lifespan that Sheik was the very best character, and Zelda the very worst. Yay for the removal of Down B transformations!!
-Ganondorf and Toon Link being slowly replaced in the modern day Zelda games. Younger Link take more after the classical Link, as shown in A Link Between Worlds, and Ganondorf hasn't had a proper appearance since Twilight Princess. Demise was the closest thing to a modern day 'Ganondorf', though Ganon is still ever present. Ganondorf, Toon Link and Sheik are mostly there because of their role in ONE game... Considering Ganondorf wasn't even originally planned to be in Twilight Princess, and was shoe-horned in, they might simply run out of ideas of make use of Ganon's human form. In a way, he's much like Sheik, famous only for his Ocarina of Time role... That, and his damn MOVE SET!!!! He's literally the only (semi)clone that takes inspiration from a character who's NOT from his home series. If that's not messy, I don't know what is.
-Hell Link and Zelda THEMSELVES might get a whole different moveset to represent Breath of the Wild. They might just cut all the Zelda characters, and give us New Link and New Zelda instead. If it takes such drastic matters to make the Zelda roster less of a total mess, I'm all for it actually! That, or they need to make new movesets for all of the characters, outside of Sheik (cause she got nothing else to work with- and she FOR NO REASON AT ALL- got more moveset changes than the other Zelda characters who needed it more! YAY FOR KEEPING SHEIK AROUND!!!!
) Toon Link might just stick around for representing the old Link move set of Smash Bros....
I get tired of thinking of the possibilities of the Zelda cast. They could also go all Hyrule Warriors with the cast, but that's somewhat less likely even though all the characters needed being present there. I also doubt we'd get a Hyrule Warriors newcomer if they gonna revamp the movesets of Ganondorf, and some other character(s) to represent Warriors. Then again, even if HYRULE Warriors isn't making a come back on Switch, Warriors does return with Fire Emblem Warriors. So it's still a sign of the massive succes that was Hyrule Warriors.
So Impa still has a chance