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.
His first potential ally could be the Lich. Since the Lich is all about destruction and chaos, just like Evil Finn in his evil state, they might team up. The Lich could provide some dark magic knowledge to Evil Finn to make him even more powerful.
Princess Bubblegum could be one. She's smart, powerful, and has a long - standing relationship with Finn. Another could be Flame Princess. Their relationship has always been complex and full of chemistry. And maybe Marceline could also be part of it. She and Finn have a great friendship that could potentially develop into more in the context of a harem fanfic.
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.
Obviously Finn himself. Since it's about him being godlike, he's the central figure. And of course, Jake his best friend is likely to be involved. Jake would probably be there to support Finn, or maybe try to keep his new powers in check if they start to get out of hand.
One possible plot idea could be Finn using his god - like powers to save Ooo from a new and powerful dark force. Maybe he has to sacrifice something precious in the process, like his friendship with Jake for a short time to fully unlock his powers.
One interesting aspect could be the exploration of Finn's new 'godlike' powers. It might show him using these powers in ways that are unexpected and exciting, like creating new lands or creatures in the Adventure Time universe.
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.