Well, one possible plot could be Finn, as a girl, having to deal with the different reactions from his friends. Some might be really supportive, while others could be a bit confused at first. Maybe he'd also discover new things about himself, like having a different perspective on relationships or challenges. For example, he could find that his usual approach to fighting monsters doesn't work the same way when he's a girl, and he has to adapt his combat style.
Finn might become more empathetic. As a girl, he could be more in tune with his emotions and those of others. For instance, he might notice small details about how his friends are feeling that he didn't before. He could also develop a different sense of style and self - presentation, which would be a new aspect of his character growth.
Another possibility is that Finn - now a girl - discovers new powers or abilities that were hidden before. Since in many stories, a change in gender can sometimes bring out different aspects of a character. She could use these newfound powers to fight against the villains in Ooo in a more creative way than before.
Well, Finn could start to notice how differently people treat her as a girl. Some might be more protective, while others might underestimate her strength. This contrast in treatment could fuel her determination to prove herself.
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.
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.