Maybe in a fanfic, Finn's death could be part of a time - traveling accident. He could get caught in a time rift while trying to save someone or something important. His death would then cause a chain reaction in the timeline of Adventure Time. The fanfic could focus on how the other characters try to fix the timeline to bring Finn back or at least come to terms with his loss.
Some write them to explore the darker and more emotional aspects of the Adventure Time universe. Finn is a beloved character, and his death can bring out strong emotions in both the writer and the reader.
Jake would be devastated. He and Finn are best friends. Jake might become withdrawn and less likely to go on adventures. He could also become overprotective of Finn's memory. Princess Bubblegum would likely be deeply affected too. She might feel guilty if she couldn't use her scientific knowledge to save Finn. She could also be motivated to find a way to bring him back. Marceline would probably be very sad and angry. Her relationship with Finn was special, and his death could make her more cynical about the world.
One idea could be exploring how the other characters like Jake deal with Finn's death. Maybe Jake would go into a deep depression at first, as they were such close friends. He might then try to find a way to bring Finn back, leading to all sorts of magical and dangerous adventures.
The Fire Kingdom's Flame Princess could be an ally too. Her mastery over fire could complement Finn's powers in combat situations. Also, the various magical creatures in Ooo, like the gnomes or the fairies, could band together and support Finn. They might have their own ancient knowledge or magic that could be useful to Finn in his adventures with his godlike powers. Finn's kindness towards them in the past might make them more inclined to help him out.
Well, in fanfics regarding Finn's arm in Adventure Time, some might focus on a magical or mythical origin for his arm. For example, it could be revealed that his arm has a connection to some ancient power or being from the Land of Ooo. It could also be used as a plot device to show Finn's growth. If his arm is injured, he might have to learn to adapt and become stronger in other ways, which would be an interesting character development arc in the fanfic.
One idea for an Adventure Time Finn arm fanfic could be that Finn discovers his arm has the ability to phase through solid objects. This new power could lead him on a quest to find out where it came from and how to control it. He might have to visit some of the more mysterious places in Ooo, like the Crystal Dimension, to get answers.
One aspect of 'Adventure Time Finn Alone Fanfiction' could be exploring Finn's character development in isolation. Finn is usually a very social character in the main series, so seeing him alone can show a different side of his personality, like his self - reliance and how he copes with solitude.
In some Adventure Time fanfictions, Finn might be betrayed by a character he trusted deeply, like Jake. For example, Jake could make a deal that goes against Finn's values in order to protect the Candy Kingdom, leaving Finn feeling betrayed as he sees it as a moral compromise. Another instance could be a new character in the fanfiction who befriends Finn only to steal his magic sword for their own gain.
Well, Finn's real story is that he's a hero in the Land of Ooo. He's constantly on the move, exploring different parts of the land. He was raised in a rather unique environment, which has made him brave and curious. Finn has this unwavering sense of justice. For example, he'll always stand up to bullies like the Lich. His relationship with Jake is a big part of his story too. They're like brothers, always having each other's backs in any dangerous situation that they encounter during their adventures.