One idea could be that Finn is kidnapped by an old enemy who wants to use him as a bargaining chip against Princess Bubblegum. Maybe they demand her to give up the Candy Kingdom in exchange for Finn's safe return.
First, establish the kidnapper's motives. Is it for revenge? Power? Or something else? For example, if it's for revenge against Finn's heroic deeds, it gives a strong starting point.
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 idea could be Marceline and Finn teaming up to explore a new and mysterious part of the Land of Ooo. They could encounter a powerful and ancient magic that they have to figure out how to control. Another plot could be Finn trying to help Marceline reconnect with a long - lost friend from her past. Maybe there's a feud between this friend and Marceline that Finn has to mediate.
A possible plot could be that there's a magical artifact deep in the Adventure Time Canyon that Finn is tasked with retrieving. However, the Canyon is guarded by a powerful being. Finn has to prove himself worthy to the being in order to gain access to the artifact. Along the way, he learns about the history of the Canyon and its significance in the Adventure Time 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.
One possible plot idea could be Finn using his god - like powers to save Ooo from a new and powerful dark force. Maybe he has to sacrifice something precious in the process, like his friendship with Jake for a short time to fully unlock his powers.
One plot idea could be that Finn is framed for stealing the Ice King's crown. He then has to go on a journey to find the real thief to clear his name. He teams up with Jake, and they start by investigating the last place the crown was seen.
One idea could be Dark Finn's struggle with his new identity. He might have to face his former friends who are now scared of him. Another could be his journey to find a way to reverse his dark state, leading him through strange and dangerous lands.
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.