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 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.
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 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.
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 Finn and Jake fanfiction could be about their adventures in a new, uncharted land within the Land of Ooo. Maybe they discover a tribe of magical creatures that they have to befriend to get a special item for Princess Bubblegum.
One interesting Finn McMissile fanfiction could be about his adventures before the events of the movie. Maybe he's on a top - secret mission in a far - off land, dealing with new villains and using his cool gadgets.
In some fanfictions, Bella and Finn might be in a different time period, like the Victorian era. They could be part of a mystery, perhaps trying to solve a crime in their small town. Bella might be a seamstress and Finn a local detective's apprentice, and their paths cross during the investigation which leads to a deeper connection.
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 interesting plot could be Fionna and Cake going on a quest to save their world from a new, mysterious dark force. They might encounter strange new creatures and have to use their wits and friendship to overcome challenges.
One interesting Finn and Rachel baby fanfiction could be about their journey in raising a musically - gifted child. The story might focus on how they pass on their love for music to the baby, and all the cute moments of the baby's first attempts at singing or playing an instrument.