Although Nintendo doesn't appear to be adding any new content to Super Mario Bros. Wonder any time soon, certain mods out there have been adding all sorts of weird and wonderful things to the new Switch entry.
One mod, in particular, as highlighted by Nintendo Everything is a new "Women of Wonder" mod created by 'Dytser' that adds in Rosalina, who made her first official appearance in the Super Mario Galaxy series.
This particular mod replaces the Blue Toad and gives Rosalina her very own unique transformations (for her elephant form and other transformations), replaces the Glide Cap with Luma, adds completely revamped audio to sound just like Rosalina, and matches up all the icons (from checkpoints to goalposts) with her theme.
"Rosalina joins the adventure: Experience the magic of Rosalina as she takes center stage in a 2D Mario for the first time. From her iconic celestial abilities to her elegant demeanor, Rosalina adds a touch of wonder to your gameplay."
This same mod series also mentions how it intends to eventually swap out Yellow Toad for Pauline if "things go according to plan". Of course, this isn't an official update or option if you're playing with a regular copy of Super Mario Bros. Wonder on a standard Switch, so just keep that in mind.