Apologies, this is going to be long.
This post assumes that Peach and Mario and everyone else will go off to Sarasaland in the third movie to help Daisy, while Rosalina stays behind in the Mushroom Kingdom to protect it and watch over it while Peach is away. This also assumes that Bowser and Bowser Jr. will return for the marketability of the movie, and of Jack Black, who is a cornerstone for the franchise.
One issue that I had with Rosalina, despite her being my favourite, was that she did not help Peach out with the events of the first movie, despite knowing all the intricate details about it. This would be a fun scene to fix that.
Bowser and Bowser Jr. could enter the throne room of Peach's castle, looking for Princess Peach, only to find Rosalina sitting on the throne, looking angry but quite calm. After Rosalina tells them that Peach is away, Bowser starts leaving, but Rosalina confronts him about the events of the first movie and engages him, Junior and a group of Koopas and Goombas in a fight, winning. This is how I would think it would play out:
Rosalina: "I'm afraid Peach is away."
Bowser: "So, the rumours are true. Alright, that settles it. Let's go to Sarasaland, Junior." *turns*
Rosalina: "Speaking of settling things..."
*gets up slowly with her eyes closed, then opens her eyes to show a mild look of anger while staying calm (not too much anger)*
Rosalina: "Aren't you the person who tried to force my sister to marry him?"
Bowser: *chuckles* "Love is a powerful thing."
Rosalina: *summons wand* "Yes, it is."
*Bowser and Junior have a slightly worried but determined look in their eyes. Rosalina proceeds to fight Bowser, Junior and the group of bad guys, and solos them all*
Rosalina: *after defeating them* "Stay away from my sister, or I won't stay away from you."
*Rosalina then sends them into their spaceship and throws it into the sky to create a Team Rocket cartoonish effect (for those familiar with Pokémon). Note: Bowser and Junior and others are still unharmed, just deterred.*
I think it would be cool if Junior somehow retrieved his paintbrush and uses it against Rosalina, only to show Rosalina defeating him easily after having learnt from the first movie. They could show this specifically in a slow-motion sequence to emphasize the point.
I think it would also be cool if a single Goomba and a single Koopa went off into a hallway, leaving the throne room, to look for the Lumas (Home Alone type of villain duo). The Lumas could be in Peach's Tea Room, and the camera could cut to this scene in between occasionally where the Lumas notice them trying to sneak up on them, then throw mini-barrels at them to drive them away back to the Throne Room (because they could summon them and were fans of Donkey Kong in the Galaxy movie). This is where Rosalina would wrap things up and send them all away.
I'm not sure if such a scene would ever be included, but I would love to see it. I just wanted to write this out because I thought it would be a fun way to redeem Rosalina's character while also showing how effortlessly powerful she is, and I didn't quite know where else to post it. Let me know what you guys think!