A great fanfiction could be set around Farmworld Finn's family. Maybe his son gets into trouble by accidentally releasing an ancient evil that was trapped in a forgotten part of the farm. Finn then has to team up with some of the local farmers, who have their own unique skills and powers, to save his son and the farm from this new threat. This would show how the Farmworld community comes together and how Finn has changed as a character in this different world.
Finn is definitely one of the main characters. In Farmworld, he has a different life compared to the regular Adventure Time universe, and fanfictions often explore his new experiences, like being a farmer and a father.
Well, in 'Finn', the characters are made memorable through their relationships with each other. Finn's interactions with his friends or foes create interesting dynamics. For example, if he has a best friend who is always getting him into trouble but also helping him out of it, that kind of relationship makes both characters stand out. Also, the character development throughout the story is key. A character who starts off as shy and then becomes bold over time is very memorable.
The Unti Finn novel by A. J. Finn is a complex mystery that weaves together multiple plotlines and keeps the reader guessing until the very end. It's full of unexpected twists and turns.
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.
He might meet some of the more reclusive characters in Ooo that we don't usually see in the show. Maybe a hermit - like wizard who gives him some cryptic advice.