I'm going to add something so that you can better understand why I say anything is possible and you never know; if you've been paying attention for at least the last few Smash games, you should know that trying to predict things is almost pointless. There's plenty of things that made sense that didn't happen and plenty of things that seemed so ridiculous and crazy but it still ended up happening.
Things that made sense enough that looked like it could have happened but unfortunately didn't:
Koei Tecmo characters - Long collaborative history with Nintendo on various games (Metroid: Other M, Hyrule Warriors, Fire Emblem Warriors) popular long-running series, and characters. Obviously, you'd think this would mean that a Koei Tecmo character would be in the game.
But what ended up happening?
Not a playable character in the roster. Instead, they got an AT from a series they co-owned with Nintendo and nothing from either of their most popular series at all.
Ubisoft characters - Collaborative history with Nintendo (Mario & Rabbids). Rayman and other characters from his series also appear as trophies in Smash Wii U. Obviously, you'd think Rayman seemed destined to be a playable character in the next Smash game.
But what ended up happening?
Not a playable character in the roster. Instead, he became a spirit, the Rabbids became spirits, and Altair (from Ubisoft's other popular series Assassin's Creed) became a Mii costume.
Travis Touchdown - Suda51 has been a supporter for the Nintendo consoles since the Wii with the No More Heroes series. The games have always been made for the Nintendo consoles first before they go to anything else and the idea of Travis being in Smash has been brought up numerous times. The newest game even references the Smash series.
But what ended up happening?
Not a playable character in the roster. Instead, Travis became a Mii costume and he had a spirit event.
Arc System Works characters - Despite not having a collaborative history with Nintendo, it does own some highly influential series (Guilty Gear, Blazblue, Double Dragon, River City) that have been on their consoles. In fact, in the cases of Double Dragon and River City, both series have been on Nintendo consoles for a really long time, as far back as the NES era. And with Sakurai mentioned Smash being a celebration of video games when referring to the game's genre (yes, people, he meant the genre, not series or characters), it would no doubt make sense for him to add the beat'em up genre to the mix.
But what ended up happening?
Not a playable character in the roster. Instead, River City got a spirit event.
Then there are things that sounded too ridiculous to happen but surprisingly did anyway:
Everyone is Here - That's pretty obvious. Literally everyone from the series was back. This meant Snake, a third party character that was missing from the previous game, was back. This meant both Toon Link and Young Link were in the same game. That sounded too good to be true...but it was true.
Microsoft characters - People want Banjo and Kazooie but also know that they're owned by Microsoft, who are competing with Nintendo. They weren't in Smash even back when Rare was owned by Nintendo, so their chances seemed non-existent.
But what ended up happening?
They became playable characters in the first season pass. By the second season pass, they were joined by Steve from Minecraft, another Microsoft series.
Final Fantasy VII content - Even though Cloud was in Smash 4, he came with very little content with him with just 2 FFVII tracks, one stage, and nothing else. That also continued for most of Smash Ultimate. Fans were ready to accept that trying to get more would have been too much for Nintendo to deal with and that Cloud was going to be stuck with just those 2 tracks, one stage, and nothing else.
But what ended up happening?
Additional SE content got added, which included another character from FFVII, another stage from FFVII, and more tracks from FFVII.
Sora - A character that was talked about pretty much all the time but nobody thought had much of a chance of getting in...and for a long time, he didn't.
But what ended up happening?
Look who the final character ended up being in Ultimate. I don't think there's really much else to say.
So this is why I say anything is possible and you never know because that's what Ultimate should have shown everybody by now. Just because some makes sense and sounds logical, doesn't mean that it will happen. Just because some sounds ridiculous and crazy, doesn't mean that it won't happen. So trying to argue stuff like that so seriously is a complete waste of time and energy. Just accept that nobody but Nintendo knows.