Finn's death would have a huge impact on Flame Princess as well. She had a complex relationship with Finn. His death could make her more reclusive, or it could fuel her determination to carry on Finn's sense of justice. The whole kingdom of Ooo would be in mourning. Finn was a symbol of hope and adventure. His friends would have to find new ways to interact with each other and deal with the threats to Ooo without him. Some might even form new alliances or rivalries in the process of filling the void he left.
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 Jake might become withdrawn and less likely to go on adventures.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.
For the Ice King, Finn's death could be a turning point. He might realize how much Finn meant to the balance of Ooo. Finn was often the one to stop his more villainous antics. Without Finn, the Ice King could either become more of a menace or, in a strange way, try to become a better person in Finn's memory. The other characters in Ooo, like the Candy People, would be in a state of shock. Finn was their hero, and his death could make them lose hope until his friends step up to restore it.
Finn might initially go into denial. He could keep expecting Jake to just pop back up like he always does. Then he'd probably get angry, at himself or at whatever caused Jake's death. Eventually, he'd start to accept it and try to honor Jake's memory by being a better hero.
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.
In the original series, Finn is alive and has all his adventures. But in the fanfiction about his death, the whole tone is much darker. The original is full of light - hearted moments, while the fanfiction is likely to be more solemn and sad.
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.
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.
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.
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.
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.
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.