Let's start with the statement of purpose: Disney Princesses have been getting cooler. Time was, their main job was to stand around and sing to woodland creatures and do the housework while the prince got on with the job of fixing whatever needed to be fixed in Fantasyland, but these days it's only suckers who wait for someday when their prince will come. These days, Disney Princesses have agency, they can fight...heck, they even have superpowers!
...wait a second...
I want to see "Disney's Avengers". Team up Merida, Rapunzel (she'd need to get her magical hair back, but hey, how hard can that be), Elsa, Mulan and Ariel (who would get her tail back when she went into the water, same as Rapunzel gets her magical hair back, because this is a reconceptualization of them as a team of superheroes and that means Ariel gets to be the Aquaman/Namor figure) to fight together against an epic Disney villain. (Or villains--maybe we could see an evil cabal of history's darkest sorcerers, Ursula, Jafar, Facilier, Maleficent and Gothel?)
Every second of this sounds like it would be awesome. The only way I could see to make it better would be to make it a time-travel story (yes, those five characters probably aren't in exactly the same time period, but they're close enough to make it fit if you don't sweat the details) so that you can bring in Lilo and Stitch for the grand finale. Because while Lilo is technically not a princess, I'm pretty sure she would tell you she was one if you asked, and her dog would be happy to back that up with all six fists.