Well, they could start by having a bit of a misunderstanding. Finn might accidentally break a water - related tradition in the Water Princess's kingdom. This causes some tension between them at first. But then, they discover a common enemy that is threatening both the Land of Ooo and the Water Kingdom. So they have to put aside their differences and team up. They could go on a journey through different elemental regions, like a fire region where they have to use the Water Princess's powers cleverly to survive.
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 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 Marceline and Finn teaming up to explore a new and mysterious part of the Land of Ooo. They could encounter a powerful and ancient magic that they have to figure out how to control. Another plot could be Finn trying to help Marceline reconnect with a long - lost friend from her past. Maybe there's a feud between this friend and Marceline that Finn has to mediate.
A possible plot could be that there's a magical artifact deep in the Adventure Time Canyon that Finn is tasked with retrieving. However, the Canyon is guarded by a powerful being. Finn has to prove himself worthy to the being in order to gain access to the artifact. Along the way, he learns about the history of the Canyon and its significance in the Adventure Time world.
One possible plot idea could be Finn using his god - like powers to save Ooo from a new and powerful dark force. Maybe he has to sacrifice something precious in the process, like his friendship with Jake for a short time to fully unlock his powers.
One plot idea could be that Finn is framed for stealing the Ice King's crown. He then has to go on a journey to find the real thief to clear his name. He teams up with Jake, and they start by investigating the last place the crown was seen.
One idea could be Dark Finn's struggle with his new identity. He might have to face his former friends who are now scared of him. Another could be his journey to find a way to reverse his dark state, leading him through strange and dangerous lands.
One idea could be Finn and Huntress Wizard going on a quest to find a rare magical artifact that can save Ooo from a new threat. Another could be a story where they get lost in an uncharted part of the Land of Ooo and have to rely on each other's skills to survive. Maybe a plot where Huntress Wizard tries to teach Finn some advanced magic techniques, but Finn has trouble grasping them at first.
The main characters are Finn, a brave and adventurous human boy, and the Water Princess. Finn is known for his heroics and his sword - fighting skills. The Water Princess has control over water and is from the Water Kingdom.
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.