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.
Another interesting idea is to do a time - travel fanfic for your Adventure Time FRT. Finn could accidentally go back in time and meet a younger version of his parents. This could lead to a whole series of events where he has to be careful not to change the future too much, while also trying to understand more about his family's past.
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.
First, think about the characters you want to focus on. Are you more interested in Finn and Jake's adventures, or do you want to explore the storylines of Princess Bubblegum or Marceline? Once you've decided, come up with a simple plot idea, like a new threat to the Land of Ooo that only your chosen characters can stop.
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 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.
You could write a fanfic about how the Mushroom War affected the magical elements in the Adventure Time world. Maybe the war disrupted the balance of magic, and certain characters are on a quest to restore it. They might encounter strange new magical creatures or phenomena that were born out of the chaos of the war.
One plot idea could be that Finn and Jake from Adventure Time end up in the Fallout wasteland. Finn, with his sword skills, could be on a quest to find a special artifact to get back to their own world. Jake, with his shape - shifting abilities, might help them survive against the various mutated creatures in the Fallout universe. Another idea is that the Vault - Tec technology in Fallout somehow merges with the magical elements of Adventure Time, creating strange new hybrids of mutants and magical beings.
One idea could be Finn and Jake swapping bodies. Finn in Jake's body might struggle with Jake's more laid - back and sometimes lazy attitude towards things like guarding the Tree Fort. He could be shocked at how comfortable Jake is just lying around. Another plot could involve Princess Bubblegum and Marceline swapping. Princess Bubblegum, in Marceline's body, has to deal with Marceline's vampire instincts like craving blood (in a non - too - gory way, of course). This could lead to some comical situations as she tries to resist her new urges while Marceline, in Princess Bubblegum's body, has to handle the responsibilities of ruling the Candy Kingdom and might be a bit too carefree for the Candy People's liking.
Another interesting fanfic idea is that Marceline loses her vampiric powers and Marshall Lee has to help her adapt to a more 'human' or at least non - vampiric life. This would lead to a lot of character development as Marceline struggles with her new identity and Marshall Lee learns more about her true self beyond her powers.