You'd probably be smart to look at Hans Christian Anderson or the Grimm Brothers Fairy Tales for the next movie. Most of the princess movies have been adaptations of stories from those two places. Something I can see being done is a take on Little Red Riding Hood. They tackled it a bit in Once Upon A Time, but maybe an animated version could turn into a princess story. They haven't stayed exactly true to the original tales, so it can be adapted so that the girl is a princess. I also wouldn't be surprised to see historical stories ala Pocahontas or Mulan. Two that might work into an adaptation could be Joan of Arc and Amelia Earheart. Might be a little tougher to do those, but I'm sure a fine story can be made. Something I'd really like to be adapted would be the story of Salvatore Giuliano. It would obviously needed to be lightened up, and a princess character would have to be written into it, but I think that is a fantastic story, and it would provide something for girls and boys. Mario Puzo fictionalized the story in his book The Sicilian. Like I said, it would have to be cleaned up quite a bit, but there is a good skeleton of story in there. Plus, it would give them another 'Epcot Princess' that they can put out for Meet & Greets in the Italy Pavillion.