I don't think Pauline's chances are all that high. Her appearance in Super Mario Odyssey will get her into the consideration phase, but unless Sakurai gets a whole lot of inspiration for her, she doesn't have anything else going for her, and her competition is steep.
I'm starting to question how steep her competition is frankly, because I'm unfortunately not all that convinced that Toad's chances have drastically improved and while Paper Mario is feasible I wonder if there would be hesitation to start adding more alternate versions of existing characters. Paper Mario has kind of been on a roll recently, so I dunno. Out of the more standard choices he might be one of the stronger ones, I guess he's in the running. Geno doesn't count as far as I'm concerned, he'd be judged independently of his role in the Mario universe. Assuming these characters are added for their mass appeal, he doesn't really fit into the puzzle.
I'm also disqualifying Waluigi, because I strongly believe he will be in the game. And there's sort of an unspoken rule, at least in my mind, that we are likely to see at least two Mario / Marioverse characters per Smash game. Every other game before this one lends itself to this and assuming next roster will have a normal amount of 12-15 newcomers, I'd consider it a statistical likelihood. Waluigi plus one feels like the move, and with upside down L being the clear fan favorite I think the second choice might skew on the side of experimentation and inspiration. And my rule of thumb is if I can think of it, Sakurai definitely can.
So under that guise I think Pauline, or King Boo, are strong candidates that offer more abstract but potentially unique moveset potential. I don't like comparing Pauline to Rosalina at all, since Rosa is far more important to Galaxy as a whole, but in this context I will - she is a character built from creative liberties. She takes elements from Mario Galaxy, but few that we actively see her perform and they are interpreted in creative ways here. All it took for Sakurai was to see Rosalina in one game and craft a puppeteer moveset... she was cited as "from Mario Galaxy" in the project plan, and it predated any knowledge of something like 3D World. So I feel like this is not such a barrier when Pauline's role and the time you spend with her in Mario Odyssey is such a standout moment of that game.
I'm not totally convinced Pauline will be playable. But I think she's pretty comfortably sitting among the top three candidates not named Waluigi, and I think we are highly likely to see a second Mario character from that crop make the jump over to playability also.