Artist Karrie Fransman and her husband, IT guy Jonathan Plackett, collaborated on Gender Swapped Fairy Tales. He devised an algorithm to swap the genders of characters in fairy tales. She illustrated the results. To learn how two creative people work together, check out the video at the link above. It's charming and just might stimulate your own work.
For instance, how would you tell a story with swapped genders? Fransman and Plackett relied on old, out-of-copyright versions of classic fairy tales for their conversions. A different approach would be to ask, what would happen if a princess found her way into a castle in which everyone had been asleep for a hundred years or a king with a magic mirror were jealous of his wife's son, Albus? Then, instead of following the received story, make it new. Imagine your characters the way you would any you were inventing. See where it takes you. Give the girls-turned-boys agency, but don't make them aggressive little monsters. Make the boys complex.
Move away from fairy tales. What if poverty-stricken John Eyre, just out of school, had to take a job with scornful Mrs. Rochester who has a secret in the attic? No need to write a whole novel, just play around with it.
Look hard at your own work in progress: What happens if you change your central character's brother into a sister?
In any case, you will not be surprised to learn that Fransman and Plackett have moved on to Gender Swapped Greek Myths! Pair it with the D'Aulaires Book of Greek Myths, throw in a Rick Riordan novel and you'll have a stellar package for an incredibly lucky child at Christmas.