Well, a fanfic could focus on Hot Dog Princess' origin story. How did she become the Hot Dog Princess? Was it a magical transformation or was she born into it? Or perhaps a story where she teams up with Finn and Jake to solve a mystery in the Land of Ooo that somehow involves hot dogs. It could also be about her trying to introduce hot dogs to a part of Ooo that has never heard of them before.
One idea could be a story where Hot Dog Princess goes on a quest to find a magical ingredient to make the best hot dogs in all of Ooo. Another could be about her making friends with a new character who doesn't understand the concept of a hot dog princess at first. Maybe a fanfic where she has to defend her hot dog kingdom from an invasion by some sort of food - hating villains.
Obviously, the Hot Dog Princess herself is a main character. Finn and Jake might also be main characters if the fanfic is set within the broader Adventure Time universe and they interact with the Hot Dog Princess. Other possible main characters could be the villains or sidekicks that are specific to the Hot Dog Princess' story, like her royal hot dog guards or an evil chef who wants to steal her hot dog recipes.
One idea could be that the Water Princess discovers a hidden underwater kingdom that was once part of her domain but has been lost. Another could be her facing a drought in the Land of Ooo and having to use her powers in new and creative ways to bring water back. Maybe she has to team up with Finn and Jake to fight an enemy that is polluting the water sources.
One idea could be a fanfic where Finn and Jake discover a new, mysterious land full of strange creatures that have powers they've never seen before. Another could be a story about Princess Bubblegum's early days as a ruler, like how she built the Candy Kingdom from the ground up. Or perhaps a fanfic that focuses on Marceline's relationship with her father and how it affected her growth as a vampire.
You could create a fanfic where Finn and Jake have to go on a mission to a completely new and unknown land in the Adventure Time world for your FRT fanfic. They might encounter strange new creatures and have to use their wits and powers in ways they haven't before. Maybe they find a land where the rules of magic are different, and they have to adapt quickly.
A great adventure time genderbend fanfic idea could involve Lumpy Space Princess as a male. He could be really vain in a different way, perhaps more focused on his muscles or some other masculine trait. And for Flame Princess, if she were a male, his relationship with Finn would be really different. Instead of the love - hate relationship they had, it could be more of a bromance, with them competing in feats of strength or something similar.
One idea could be that the Water Princess discovers a hidden underwater kingdom that has a connection to the Land of Ooo. She has to team up with Finn and Jake to protect it from an evil sea creature.
One idea could be that the Ice Princess decides to explore the human world for the first time. She might encounter strange things like cars and skyscrapers and have to figure out how to adapt. Another could be a story where she teams up with Finn and Jake to save the Ice Kingdom from a new, mysterious threat that is melting all the ice. Maybe she also discovers a long - lost power within herself during this adventure.
One idea could be that Breezy gets lost in the Land of Ooo and Finn and Jake have to help her find her way back home. Maybe she has a special mission that she needs to complete but can't without their help.
One idea could be a story where Fionna and Cake find a magical portal that leads them to a parallel universe where all the characters have swapped genders. It would be a really fun and unique take on the Adventure Time world. Another idea is that they go on a quest to find the rarest ingredient for a cake that has the power to grant wishes. They could face many challenges along the way, like battling new and strange monsters.
A fanfic could explore the relationship between Ooo and Aaa more deeply. Maybe they get into an argument over how to handle a new threat in Ooo, and through their differences, they learn more about each other and strengthen their friendship.