One idea could be Finn going on different adventures with each member of the harem separately, and then they all come together to face a big threat in the Land of Ooo. For example, he might go on a treasure hunt with one, a quest to save a magical creature with another.
Well, a plot could involve Finn accidentally creating a harem situation. Maybe he saves a group of girls from a common enemy and they all become smitten with him. Then they follow him around on his adventures, and Finn has to learn how to deal with all their different personalities and needs while still being himself and doing good in the Land of Ooo.
One plot idea could be Finn and Huntress Wizard going on a quest to save a magical forest. They encounter various strange creatures along the way and have to use their combined skills to overcome obstacles. Another could be a story where they get trapped in an alternate dimension and have to find their way back home while dealing with the unique challenges of that dimension.
One plot idea could be Finn and Huntress Wizard going on a quest to find a magical artifact that was lost in the Enchiridion. They face various challenges along the way, like battling strange creatures and solving riddles.
Princess Bubblegum could be one. She's smart, powerful, and has a long - standing relationship with Finn. Another could be Flame Princess. Their relationship has always been complex and full of chemistry. And maybe Marceline could also be part of it. She and Finn have a great friendship that could potentially develop into more in the context of a harem fanfic.
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 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.
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.