One common theme is self - discovery. Fern is often portrayed as trying to figure out who he really is, especially considering his unique origin in the Adventure Time universe.
There might be a fanfiction where Fern goes on a solo adventure away from Finn and Jake. He encounters new creatures and has to rely on his own skills, which he's still developing. This could show his growth and how he adapts to different situations without his 'creator' Finn by his side. In the process, he might make new friends or enemies.
One great 'Fern Adventure Time fanfiction' is 'Fern's New Beginnings'. It explores Fern's character development further in new and exciting scenarios. It gives more depth to his relationships with other characters from the show.
Archive of Our Own (AO3) is a great place to find Adventure Time Fern fanfiction. It has a large collection of stories written by different authors from all over the world. You can search for Fern - related fanfics easily using the search bar.
One interesting fanfiction could be where Finn's death is a self - sacrifice to save Ooo. In this story, he might face an unbeatable enemy and choose to give his life to protect his friends and the land he loves. Another could be a more emotional take where Finn dies of an incurable disease, and his friends have to come to terms with his loss and remember all the good times they had with him.
There could be a fanfic where Finn's death is a sacrifice to save Ooo from a great evil. In this story, his death might not be in vain as it inspires others to be braver. Maybe Marceline, who has seen a lot of loss in her long life, becomes a mentor to the younger characters as they cope with Finn's death, teaching them about the importance of memory and how to keep Finn's spirit alive.
It can be quite divisive. Some fans who are very attached to Finn may be really sad or even angry about the idea of him dying in fanfics. They might see it as a negative take on the character they love. However, others may appreciate the exploration of deeper emotions and different storylines that such a concept brings. It allows for more complex character development for the remaining characters as they deal with grief.
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.
Well, perhaps in the fanfic, Jake's death could lead to a change in the balance of power in Ooo. Evil forces that were previously held at bay by Jake's strength start to emerge. Maybe the Ice King, instead of seeing Jake as an obstacle to his schemes regarding Princess Bubblegum, now has free rein. Finn would have to step up in a big way, but without his best friend, it would be extremely difficult. He may struggle with feelings of guilt, thinking he could have saved Jake.
It can also make readers feel reflective. His death in a fanfiction can make people think about the character's journey throughout Adventure Time. They might start to think about all the things Finn has accomplished, the relationships he's built, and how his death impacts the world of Ooo and his friends.