Alright, guys, I got something else in mind for Rosalina that should've been in the movie.

This is a good one.

She could've just stayed in the cage, instead of being overpowered by that machine, they could've just used the Grand Star to power up the Boomsday Weapon (just like in Sonic, Eggman uses the Chaos Emeralds or Master Emerald to Power Up the Eclipse Cannon).

And Peach fights Kamek to rescue Rosalina, let's just say after Kamek warns Bowser and Bowser Jr that Mario and his friends are in Space Junk Galaxy AKA Planet Bowser, he has been told to watch over Rosalina til Peach gets there, this is where Kamek gets a role, Kamek takes Rosalina's power which overpowers her, and all that power Kamek took gave him power.

When Peach finds Rosalina in a cage, Kamek appears all mighty and powerful, Peach demands that he released her sister at once but Kamek refused, the power is all his now, and in a few minutes, the Boomsday Weapon will fire, she gonna have to fight Kamek to free Rosalina and stop that machine. 

Peach tries her best to fight Kamek, but Kamek knocks her down, but Peach didn't give up, she came all way to find her sister, and she's way too far to give in, she came right back up and continues to fight, she dodges every one of Kamek’s attacks, magic, minions, even Koopa shell. She uses thoses shells to her advantage, she throws and reflects them at Kamek few times (just like the games), then after one final hit, Kamek loses all the Power and returns to his normal self unconscious, then Kamek’s wand fell in front of unconscious Rosalina, broke and releases all of her power and energy, Rosalina gains her power and energy, and broke out of the cage, Peach and Rosalina ran and hugs each other shortly (they don't have time for the reunion yet),

Then they work together to stop the Boomsday Weapon, they tried their best to get the Grand Star out of there, but the glass is too strong, but Peach had an idea, she tells Rosalina to touch her hand, Rosalina knows what she's talking about, she reaches out her hand to Peach's hand, then they closed their eyes to focus, the magic releases, and their other hands touches the glass, as the magic grows, the glass begins to crack, the power destroys the glass, the Grand Star is out, and the power transformed Planet Bowser into a nature-filled garden, then they hugged again as a reunion.

Rosalina needs more screen time Part 2