It could start with Finn exploring a strange new area in his world and suddenly stumbling upon a portal. When he steps through, he finds himself in Fionna's world and right in front of her. There's an immediate sense of confusion and curiosity from both of them.
One interesting fanfiction could be where Finn and Fionna meet in a magical forest. They might initially be cautious of each other but then realize they have a lot in common, like their adventurous spirits. They could team up to defeat a common enemy, perhaps an evil wizard who has been causing chaos in both their worlds.
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.
The plot might involve Finn and Jake being initially confused by the different gender - swapped versions of themselves in Fionna and Cake's world. Then they learn to work together to solve a mystery that is threatening both of their worlds. It could start with Finn and Jake following a strange portal that leads them to this new place with Fionna and Cake. As they interact, they find that their skills and personalities are both similar and different from their counterparts, which makes for an interesting dynamic.
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 appeal lies in the unique combination of their characters. Finn's adventurous spirit and Marshall's laid - back vampire charm create an interesting dynamic. It's fun to read about how they interact in different scenarios.
One great Finn x Marceline fanfic is 'Adventure in the Night'. It beautifully explores their relationship in a post - apocalyptic setting. The author does an amazing job of portraying Finn's growth and how he and Marceline interact in this new world.
I really like 'Finn and Marceline's Unlikely Alliance'. It shows a unique take on their relationship where they start off as reluctant partners but end up having a strong bond. There's also 'Marceline's Song for Finn' which is a more emotional fanfiction that focuses on Marceline's feelings towards Finn through her music.
I really like 'Finn and Marceline's Night Out'. In this fanfic, they have a sort of double - date situation with Jake and Lady Rainicorn which leads to some really funny and heartwarming moments. It shows a different side of their relationship in a more comical setting.