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.
One interesting fanfiction could be about Finn and Jake going on a quest to a mysterious new land that wasn't explored in the show. Maybe they encounter a new type of magical creature that challenges their friendship and they have to work together to overcome it.
Well, it could be about the kids going on their own crazy adventures. Maybe they inherit Finn's bravery and Jake's shape - shifting abilities and use them to save the Land of Ooo from a new threat.
One interesting fanfiction could be about Finn and Jake going on a new, never - before - seen adventure in a strange magical land. Maybe they encounter a new type of magical creature that challenges their friendship and they have to work together to overcome it.
It could be an old enemy of Finn and Jake who has come back to take revenge on their kids. Maybe the Ice King's descendants or something related to him.
You can start by checking out popular fanfiction websites like Archive of Our Own. Just type in 'Adventure Time Finn x Jake' in the search bar and you'll get a list of relevant stories. Look for ones with high ratings and a lot of positive reviews.
First, think about the characters. Know Finn's adventurous spirit and Jake's laid - back but loyal nature. Then, come up with an interesting plot. It could be something as simple as them finding a lost treasure in the Land of Ooo. Next, write in a style that matches the show, with a bit of humor and lots of action.
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.