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 Lich could be a great antagonist. He might frame Finn as part of his plan to spread chaos and death in Ooo. Since the Lich is always looking for ways to disrupt the peace, using Finn as a scapegoat would be a clever move for him.
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.
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 main characters are clearly Finn and Jake from their own world, and Fionna and Cake from the alternate world. Finn is brave and adventurous, always ready for a fight. Jake is more laid - back but also very loyal. Fionna is a spunky girl with her own set of skills, much like Finn. And Cake, the talking cat, provides comic relief and is also very resourceful, just like Jake in many ways. They are all important as they interact and their stories intertwine in this fanfiction.
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.
Princess Bubblegum could be a part of the fanfiction. She might be interested in the unique flora and fauna of the Canyon and could send Finn on a mission there. Marceline the Vampire Queen could also be added. Maybe she has some history with the Canyon and her presence could add an element of mystery and danger.
It could be a remnant of the Ice King's past enemies. Maybe one of the magical creatures that the Ice King used to annoy or fight against now sees Simon as a threat. Since Simon has the knowledge and power of the Ice King, this creature wants to stop him before he becomes too powerful again.
It could be another powerful time - manipulating ghost. This ghost might see Danny as a threat to his own plans of ruling over all time periods. He would try to stop Danny from fully understanding and using his new powers.
It could be an old enemy of Finn and Jake who has come back to take revenge on their kids. Maybe the Ice King's descendants or something related to him.