You know, I had my own thought on a game that focused on Rosalina.
The story first starts off at a galaxy that's far, far away. That very galaxy was home to a peaceful kingdom, which had a king, queen, and their son, a young prince. One day however, an evil force attacks the kingdom. During the attack, the king and queen placed their son in a spaceship, in hopes of finding someone who can save their kingdom.
The story then switches to the Comet Observatory, where Rosalina is serving dinner for her Lumas. However, the dinner soon gets interrupted when Polari notifies Rosalina about a stranded spaceship. Rosalina checks it out, but soon hears an odd voice. The voice apparently came from inside the spaceship, so Rosalina decided to bring it onto the Comet Observatory. Upon opening the spaceship, Rosalina and the Lumas notice that someone is in there, and is passed out. It was none other than the young prince himself.
Rosalina brings the passed out prince to her bedroom, in hopes that he'll be okay. A few days later, the young prince wakes up, and is confused about where he is. Rosalina then comes in, noticing that the prince is awake. The two then make a conversation, until the prince started crying. The prince told Rosalina that he lives at a galaxy that got attacked by an evil force, and his parents had to send him away. He was unsure about what happened to them, which made him even more upset. Rosalina tries to calm the prince down, and tells him that she'll help him save his kingdom. The prince then calms down, and decides to tag along with Rosalina. And with that, the adventure begins, as Rosalina prepares the Comet Observatory to head to the prince's home.
However, it turns out that the evil force that attacked the prince's home also affected multiple worlds as well, which will require Rosalina to have to free those worlds and their galaxies first.
There are 8 worlds total, and the galaxies in them utilize the elements from Super Mario Galaxy, but also some elements from the 3D Legend of Zelda titles.
Additionally, the first 7 worlds' major bosses involve a ruler who has been possessed. Rosalina has to free those rulers by using her star powers to defeat the evil sources that are controlling them. The final boss is the leader of the evil force, and the mastermind of the invasion; the leader first starts out as a demonic humanoid, but its true form is really a dark beast, preferably a dragon.
On a side note, the prince is a small yellow dragon, who is only about as big as a Luma. And as for the prince's kingdom, it is home to the dragons of light, who all value peace and prosperity. Whereas, the evil force is known as the dragons of darkness, who all value pain, tyranny, and are very sadistic. Basically, Rosalina has to help the prince defeat the dragons of darkness, and free every galaxy that they've invaded.
A crazy idea perhaps, but since Rosalina is the guardian of the cosmos, this should really strengthen Rosalina's status.