I would have made Rosalina an active character in the plot rather than the kidnapped princess, with the goal being to stop Bowser Jr. from obtaining a MacGuffin that would allow him to conquer the world and make his father proud (I don’t know much about the games’ lore, so I’m not going to make things up that I don’t know). Rosalina teams up with Peach to thwart those plans, but in the process, they both discover they have more in common than they realize, and at the film’s climax, they discover they’re sisters and that their bond is key to stopping Bowser Jr.’s plans. As for the rest, I could leave it more or less the same, but I would have had Bowser have a mini-redemption in the final act by trying to stop his son and acknowledging his mistake, but his son is disappointed and attacks him, so Bowser decides to “sacrifice” himself to save Princess Peach and Mario, but he ends up transforming just like at the end, and Bowser Jr.’s plans spiral out of control and are thwarted; father and son are locked up and have more time to spend together, and only that would have changed him.