Well, a plot could revolve around the return of some old foes in a new form in the future. For instance, the Lich could come back as a more insidious threat that infects the minds of the future inhabitants of Ooo. Also, we could explore the idea of Finn's sword having new powers or a different significance in the future. Maybe it's the key to unlocking a long - forgotten dimension that holds the secrets to saving Ooo from a new impending doom.
One idea could be Shoko discovers a new, hidden realm in the Land of Ooo. She teams up with Finn and Jake to explore it and uncovers ancient secrets that could change the whole world of Adventure Time.
One idea could be a new student arriving at Adventure Time Academy who has a mysterious power that no one has seen before. This new character could disrupt the existing social dynamics among Finn, Jake, and their friends.
Maybe Marceline the Vampire Queen teams up with a Dark - type Pokémon in a fanfiction. Since she has a somewhat dark and mysterious personality, it would be interesting to see how she interacts with a Pokémon that shares those traits. For example, she could form a bond with a Umbreon and they could go on adventures together.
One idea could be exploring Finn's family background more deeply. Maybe he discovers a long - lost relative who has a unique power. Another could be about Jake's pups and how they grow up in the Land of Ooo, facing different challenges as a family. Or perhaps a story about Princess Bubblegum's family tree and how it affects her rule in the Candy Kingdom.
One idea could be Future Trunks bringing back a technology from the future that changes the course of the present timeline's battles. For example, a more advanced scouter that can detect hidden power levels more accurately.
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 OC is a new traveler in the Land of Ooo. They accidentally save the lives of several main characters' love interests. As a result, these characters become attracted to the OC and form a sort of harem around them. For example, Princess Bubblegum's scientific assistant could be among them, drawn to the OC's unique knowledge from another land. Another plot could involve an OC who has a special power that is needed to save the harem members from an ancient curse. This would create a lot of bonding and relationship - building moments.
One idea could be exploring the backstory of the Red Throne. Maybe it was created by an ancient and powerful being for a specific purpose that has been long forgotten in the Adventure Time universe.
A possible plot for an Adventure Time Ice Prince fanfiction might be that the Ice Prince decides to explore the warmer parts of Ooo. He's never been there before and is curious. But once he gets there, he realizes he can't control his ice powers well in the heat. He accidentally freezes some important things and has to make amends. Finn and Jake get involved to help him learn to control his powers in this new environment. It could also be a story about him learning about different cultures in the warmer regions.
A plot could revolve around Cake finding a long - lost relative. This relative could be a powerful and mysterious cat - like being who has some dark secrets. Fionna and Cake then have to decide whether to trust this newfound relative or not. Maybe this relative is being hunted by an even greater evil, and by helping, they put themselves in danger. It could also be a story where Fionna loses her memory and Cake has to take on the role of guiding her through their known world while also trying to find a way to restore her friend's memory.