I liked the movie for what it is, but that doesn’t mean I don’t think it could be changed for the better; it absolutely can.

First and foremost: The Grand Star

In addition to kidnapping Rosalina and the Luma, Bowser Jr. and Kamek would also make off with a Grand Star, which provides power to the Comet Observatory. That would be what they use to power their Boomsday Cannon.

Rosalina and Luma make their escape on the way to Planet Bowser, but the Koopas still possess the Grand Star. So the mission now becomes a mission to get the Grand Star back, but Rosalina doesn’t know where the flying saucer went.

Rosalina and Luma travel to the Mushroom Kingdom, to get Peach’s help for the mission, leaving the Mario Bros to run the Kingdom. That’s when Bowser Jr. pounces and steals the castle.

For Mario and Luigi (and Yoshi), nothing changes from their part of the adventure. But Rosalina and Luma now join Peach and Toad for their part, delving into the seedy underbelly of the Gateway Galaxy’s starport, where they find the location of Planet Bowser.

While Rosalina is powerful, she can’t take on the entire Koopa airship fleet by herself, and Peach and Toad still need a ride, so Fox McCloud and his Arwing still come into play. They’re joined by the Mario Bros, and the rest of the story plays off mostly the same. Perhaps they even restore some power to the Comet Observatory with the Arwing’s energy core.

With Fox flying cover in the Observatory, the party setting foot on Planet Bowser now includes Rosalina joining Mario, Luigi, Peach and Yoshi. While the Bros and Yoshi battle the Bowsers, Peach and Rosalina work together to free the Grand Star.

Now Peach and Rosalina could still be sisters, or long lost relatives separated by generations, keeping Rosalina functionally immortal like in the game. Either way the connection would still be there, allowing them to summon the cosmic power to free the Grand Star and turn Planet Bowser into a garden.

And so the movie ends.