Perhaps Finn is reminded of his past traumas in a fanfic. He has been through a lot, like being separated from his family at a young age. If something in the fanfic triggers those memories, it could bring him to tears. Also, if he feels betrayed by someone he trusts, that would be a very valid reason for him to cry. He is a character who values loyalty, and a betrayal would be a huge blow to his emotional state.
He could cry due to unrequited love. Finn has had his fair share of romantic interests in the show. In a fanfic, if the girl he loves doesn't return his feelings, it would be a heart - breaking situation for him. Also, in some fanfics, Finn might be confronted with the harsh reality of the world he lives in. Maybe he discovers something truly dark about the Land of Ooo that shatters his innocent view of it, causing him to break down in tears.
A writer can describe Finn's physical reactions. For example, they can write about his eyes welling up with tears that start to roll down his cheeks. Mentioning how his voice might crack when he tries to speak can also be effective. Another way is to show his body language, like his shoulders slumping as he cries.
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.
A possible plotline could be Finn getting lost in a magical forest all by himself. He has to find his way out while facing strange creatures and magical obstacles.
One reason could be that Finn has grown tired of the limitations within fanfiction. In Adventure Time, Finn is a very action - oriented character. Fanfiction might not offer him the same level of real - world exploration and adventure that he craves. Another possibility is that he wants to create his own original story outside the constraints of fan - created worlds.
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 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.
Another possible theme is the exploration of different cultures within the Adventure Time universe. Lady Rainicorn comes from a different species, and fanfictions can delve into how her culture and Finn's human - like culture interact and influence each other.
One positive theme could be Finn's adventures in new and unexplored lands. He could discover new magical creatures and form friendships with them. Another theme could be Finn's growth as a hero, learning new skills and becoming even more courageous. Also, his relationships with other main characters like Jake and Princess Bubblegum can be explored in a positive way, like how they support each other through tough times.
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.