One possible scenario in this fanfiction could be that Finn's transformation into a vampire brings out a whole new side of his personality. He might struggle with his newfound thirst for blood while still trying to uphold his heroic values. Maybe he teams up with Marceline, who is already a vampire, and she teaches him how to control his urges.
Once upon a time in the Land of Ooo, the Vampire King emerged from his dark lair. He was a menacing figure, with his red eyes and long, black cloak. Finn and Jake heard rumors of his return and set out to stop him. But the Vampire King had a plan of his own. He was tired of being feared and wanted to make friends. Finn and Jake were skeptical at first, but as they got to know him, they realized he wasn't all that bad.
There's a fanfiction that focuses on Finn's journey to save a vampire - infested kingdom. In this story, Finn teams up with some unlikely allies, including a vampire who has a change of heart. It shows Finn's determination and his ability to see the good in everyone, even vampires. The fanfiction is really detailed in terms of world - building, creating a whole new vampire - related ecosystem within the Adventure Time universe.
In the Adventure Time world, Finn and Jake decide to take a wild journey to the moon inspired by 'Fly Me to the Moon'. They build a rickety spaceship out of junk from the Land of Ooo. As they blast off, they sing the song. Once on the moon, they meet strange moon creatures that look like floating crystals. Finn tries to communicate with them using his sword as a sort of translator, which leads to some hilarious misunderstandings.
Maybe the Vampire King was once a human who was bitten by a powerful and ancient vampire. As he turned, he found himself with new powers and a thirst for blood. Over time, he became more powerful and more feared, eventually rising to be the Vampire King. His main goal was to create a vampire empire in the Land of Ooo.
The 'Finn and the Vampire Clan' is quite popular. It has an engaging plot where Finn discovers a hidden vampire clan in the Land of Ooo. The story is full of twists and turns as Finn tries to understand their motives and whether they are a threat or not.
Perhaps BMO gets curious about the moon after listening to the song. BMO builds a tiny rocket and sets off. On the moon, BMO finds a crashed alien spaceship. Inside the spaceship, there are some strange devices. BMO starts to tinker with them and accidentally activates a signal that attracts a group of space pirates. Finn and Jake, who were following BMO's signal, have to help fight off the pirates and save BMO.
Well, perhaps Finn gets bitten by a vampire while on one of his adventures in the Land of Ooo. As a vampire, he has to adapt to a new lifestyle. He might start to prefer the night and avoid the sun. His diet could completely change, which would be a big shock to his friends who are used to his normal eating habits.
A really cool fanfiction could be set in an alternate universe where Finn was born a vampire. He has to grow up in a world that fears him, but he still has the heart of a hero. He might use his vampire abilities for good, like super speed and strength, to protect those who are weaker, and in the process, he tries to change the perception of vampires in Ooo.
Princess Bubblegum could also be a main character in such fanfictions. She might be the one trying to find a scientific solution to Finn's vampirism. Her intelligence and resources in the Candy Kingdom could play a big role in the story. Another character could be Marceline. As a half - vampire herself, she could either be a mentor to Finn or a source of conflict as she has her own views on vampirism.