There's a chance that Finn would meet an ancient being, like a sentient tree or a powerful spirit. This being could have a connection to the early days of Ooo and could tell Finn stories about how the land came to be. Through these interactions, Finn could gain a deeper understanding of the world he lives in and his role in it, which could ultimately influence his decision on whether to return home or continue running away.
Finn could run into a group of nomadic adventurers. They would be different from Finn in many ways, but they could also teach him new survival skills and show him different perspectives on life. They might be a bit more carefree than Finn is used to, which could be both exciting and challenging for him.
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.
In the fanfiction, Finn could run away because he's haunted by his past actions. He may think that by leaving, he can escape his inner demons. So he sets off into the unknown, facing various challenges like dangerous creatures and harsh terrains. Along the way, he discovers things about himself that he never knew before, and this could lead to a new understanding of his place in Ooo.
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.
Mal might meet a group of outcasts like herself. They are all running from different problems but have formed a kind of family. Together, they teach Mal the importance of community and support, and they all help each other face the challenges that come their way.
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.
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.
One possible scenario in this fanfiction could be that Finn's transformation into a vampire brings out a whole new side of his personality. He might struggle with his newfound thirst for blood while still trying to uphold his heroic values. Maybe he teams up with Marceline, who is already a vampire, and she teaches him how to control his urges.
I'm not sure who the absolute best is, but some popular ones might be those who have a deep understanding of the Adventure Time universe. Writers who can capture Finn's adventurous spirit and Bear's unique personality well are usually great. For example, some authors on big fanfiction platforms who have a lot of positive reviews for their Adventure Time related works could be considered good authors.
He could meet some of Iwa's powerful ninjas. Maybe Onoki, who could either see him as a threat or an opportunity. There might also be some young Iwa ninjas who are curious about Naruto and his abilities.
He might meet some of his distant Uzumaki relatives. There could be elders or other Uzumaki clan members who have valuable knowledge about his family and their special abilities.