Well, in some fanfictions, Evil Finn could be on a quest to find a powerful artifact that would enhance his evil powers even more. Maybe it's a dark crystal hidden deep in the dungeons of some long - forgotten castle. He'd have to face many challenges and foes along the way, and this could lead to some really exciting action scenes and character development.
One possible plot could be Evil Finn trying to take over the Candy Kingdom. He might use his dark powers to turn the sweet inhabitants into his minions. Another plot could involve Evil Finn forming an alliance with other villains from the Adventure Time universe, like the Ice King in his more menacing moments. He could then plan to rule over all of Ooo together.
In this fanfiction, an exciting plot could be Si being reborn as Finn but with some of his own memories intact. He then tries to find a way to merge his old self with Finn's heroic identity. Along the way, he might meet Jake and they could go on adventures together. Si could also have to deal with the moral dilemmas that Finn sometimes faces, like whether to help a villain who seems to have a good side.
One interesting plot could be that the Jungle Princess and Finn go on a quest to find a magical artifact hidden deep in the jungle. Along the way, they face various challenges like strange creatures and tricky puzzles.
One of the most intriguing plots could be the Green Knight challenging Finn to a battle of wits rather than just physical combat. Finn, known for his straightforwardness, would struggle to outwit the cunning Green Knight. Another could be the Green Knight having a mysterious connection to the Ice King, perhaps revealing a long - lost history between the two. And a third might be the Green Knight being a guardian of an ancient and powerful magic that Jake accidentally stumbles upon, leading to a series of adventures to make amends and understand the magic's purpose.
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.
Perhaps a plot where Naruto is influenced by an evil godlike entity. This entity could be from a different dimension. It whispers dark thoughts into Naruto's mind, gradually turning him against the values he once held dear. As a result, he starts to perform actions that are completely out of character for the original Naruto, like attacking innocent villages to gain more power.
One great 'Adventure Time Finn' fanfiction is 'Finn's New Quest'. It takes Finn on a journey to a mysterious land beyond Ooo. Another is 'Finn and the Lost Crystal', where Finn has to find a powerful crystal to save the Candy Kingdom. There's also 'Finn's Friendship Trials' which focuses on Finn's relationships with his friends in unique and challenging situations.
One interesting theme is Finn's search for identity. Since he's a human in a strange land full of magical beings, he often questions who he is and where he belongs. Another theme could be his relationships, like his brotherly bond with Jake or his complex feelings towards Princess Bubblegum.
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.