Perhaps Finn, as Fionna, discovers new magical abilities that are unique to this form. He could use these abilities to fight off villains that are specifically targeting Fionna. This would lead to him having to adapt his fighting style and strategy, which was more Finn - centric before. And in the process, he might learn more about the female perspective in the Adventure Time universe.
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.
In this fanfiction, Finn as Fionna could also have a romantic subplot. He might develop feelings for a character that Fionna has a connection with, like Flame Prince. This would be an interesting twist as Finn has his own history with relationships in his normal form. And it would show how being in Fionna's body and role affects his emotional and romantic interactions.
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.
Perhaps Finn's death is an accident. He could be exploring an uncharted area of Ooo and stumble upon a dangerous trap or ancient magic that he doesn't understand. His death shocks everyone in Ooo, and his friends start to question the safety of their continued adventures. This could lead to a whole new set of adventures for his friends as they try to come to terms with his death and make Ooo a safer place in his memory.
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 popular storyline could be Finn's hair having magical properties. For example, his hair might grow uncontrollably and start to have a life of its own, leading him on an adventure to find a way to control it. Another could be a story where Finn's hair is the key to unlocking a hidden realm. Writers might also explore a plot where Finn's hair changes color based on his emotions, which then affects his adventures in strange ways.
One interesting storyline could be Finn and the lady having a deep conversation about the nature of good and evil in the Land of Ooo. Maybe they discuss how to handle the various villains they've encountered.
He could cry due to unrequited love. Finn has had his fair share of romantic interests in the show. In a fanfic, if the girl he loves doesn't return his feelings, it would be a heart - breaking situation for him. Also, in some fanfics, Finn might be confronted with the harsh reality of the world he lives in. Maybe he discovers something truly dark about the Land of Ooo that shatters his innocent view of it, causing him to break down in tears.
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.
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.
One interesting storyline could be Huntress Wizard taking Finn on a magical quest through the Land of Ooo that he's never been on before. They could encounter strange new creatures and Finn has to rely on Huntress Wizard's knowledge of magic to survive.
A possible plotline could be Finn getting lost in a magical forest all by himself. He has to find his way out while facing strange creatures and magical obstacles.