RE: Pauline
I think drawing parallels to Rosalina here is very misguided. When Rosalina was chosen for Smash 4, she barely had any spinoffs under her belt. What got Sakurai to consider her in the first place was her dynamic with Luma, which means the only game that mattered in his mind was the original Super Mario Galaxy. Her subsequent appearances in more Mario spinoffs and Super Mario 3D World came after she was chosen for Smash 4.
This means that Pauline being included in Mario Kart, Mario Tennis and the like isn't doing her any favors when it comes to Smash. Those games are great for boosting her popularity and relevance, but why would they prompt Sakurai to include her in Smash? She's one of at least 20 characters in those games. They don't make her "special".
Another point against Pauline is that she's never been an "active" character. Granted, SMG Rosalina wasn't active either, and that didn't stop her, but that comparison is false too. She was always shown to be powerful, even though she didn't outright show it. She carried a wand and was shown to be able to levitate and summon force fields. Pauline just sits there, talks to you, and sings a song while dancing (rather constrainedly) in SMO. The only other "passive" NPC that have gotten into Smash as original fighters are Sheik and Zelda, but those two: A) have clear concepts laid out already (ninja/magician) B) are a huge deal in their game of origin. Pauline's side-story in Odyssey is more of a fun distraction, even though the New Donk City festival is a stand out moment.
However, Pauline could offer a chance to bring SMO's mechanics into Smash, which Sakurai could find interesting. This is veering into Rosalina-parallel territory again, but it's a possibility. She was given a huge hat in SMO after all. I don't think she would straight up Capture people, but simple stuff like bouncing on the hat for an extra jump seems realistic. She also has a musical motif, and a very diva-like demeanor, which makes it easier to envision her "dancing" (literally?) on the battlefield. The Minis from the Mario vs DK could be included but I don't know if the moveset would get too busy with that.
Another thing in Pauline's favor is that she's never really had the chance to be considered before. Ultimate's roster was decided in late 2015, before Pauline's reintroduction in SMO. She's a brand new consideration, so we don't know how Sakurai feels about her. Characters like Toad and Geno had their chance but were passed up multiple times before, but Pauline hasn't even had the chance to be considered yet. We don't know if she will be, but so far, it's a plus for all we know.
Despite liking her a lot, I'm still on team "Pauline's unlikely". The competition is simply too stiff, and she is a character I find hard to envision, both from my perspective and (hypothetically) Sakurai's. I feel like he gravitates towards characters with an enticing hook or gimmick, that have an interesting silhouette and can be easily recognizable from their movements. Pauline was quite literally designed to be a generic human lady.
Maybe she'll have to make her Mario Party debut first...