It's interesting to compare Rosalina and Pauline. People will say Pauline is the new Rosalina, Rosalina was put in everything, so Pauline is put in everything so Smash will be next.I think she has a chance like how Rosalina got added after being a breakout character.
There are other options if it’s strictly main Mario: King Boo, Petey, and Kamek are sometimes mentioned. Koopa/Dry Bones, Shy Guy and Nabbit if it’s about choices that are considered less serious.
I still have think that Toad should be included at this point rather than just being a counter and throws. Also Waluigi.
But there's important distinction. Rosalina got into Smash before she got into everything else. All she had to her name was the Galaxy titles, Mario Kart, and very recently a playable role in 3D World. (And a Mario Party cameo).
Rosalina wasn't in Mario Tennis Open, Mario Party 9, had to wait for DLC in Mario Golf World Tour. It wasn't until after Smash she appeared in Mario & Sonic. Wasn't in Super Sluggers massive roster.
It's admittedly a lower number of spinoffs. But Pauline was in Odyssey and then immediately started being put in things, Mario Kart Tour, Mario Tennis Aces, Mario Golf Super Rush, Mario Party Jamboree, even the tiny roster that was Mario Strikers. I think she only missed out on the first two switch Mario Parties and Mario & Sonic.