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.
The plot could involve Finn's journey to find a cure for his vampirism. Along the way, he meets other vampires, some of which are friendly and some are not. He has to use his skills as an adventurer to survive in this new world of vampires. His relationship with Jake could also be strained as Jake is worried about losing his best friend to this new 'dark' side. Finn may also discover secrets about the origin of vampires in Ooo that no one knew before.
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.
One possible scenario could be that Finn, after being turned into a vampire, struggles with his new identity. He might have to deal with the urge to drink blood while still holding onto his old values. For example, he could try to find alternative ways to satisfy his thirst that don't involve harming humans, like drinking animal blood.
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.
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.
It could also be a corrupted version of one of Fionna's friends. For example, if Cake were to be turned evil by some dark magic. Cake would know all of Fionna's weaknesses and could use that knowledge against Finn in his Fionna form, making her a formidable and tragic antagonist.
One possible storyline could be Finn exploring the world as Fionna. He might have to deal with the different social dynamics in the female - dominated version of the Adventure Time world. Maybe he'll struggle to fit in at first but then find his own place among the characters like Cake and Prince Gumball.
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.
Well, in the fanfiction, Finn could be part of a secret vampire society. He'd have to learn their rules and customs. There might be a hierarchy among the vampires, and Finn would have to find his place. Maybe he'd be a special kind of vampire with unique powers that the others envy or fear, which would create a whole lot of drama and intrigue within the vampire community.
Well, one possible plot could be Finn, as a girl, having to deal with the different reactions from his friends. Some might be really supportive, while others could be a bit confused at first. Maybe he'd also discover new things about himself, like having a different perspective on relationships or challenges. For example, he could find that his usual approach to fighting monsters doesn't work the same way when he's a girl, and he has to adapt his combat style.