The Lich could be a great antagonist. He might frame Finn as part of his plan to spread chaos and death in Ooo. Since the Lich is always looking for ways to disrupt the peace, using Finn as a scapegoat would be a clever move for him.
It could be a jealous rival of Finn's. Maybe someone who is envious of Finn's heroic status. This character could plant false evidence and manipulate situations to make it look like Finn is guilty. For example, a new adventurer in Ooo who wants all the glory for himself and sees Finn as a threat.
Another option is a group of bandits. They could frame Finn for their own misdeeds. They might steal something valuable from a village and leave evidence that points to Finn. Their motivation would be to get the bounty on Finn's head and also to get the villagers off their backs. Finn would then have to hunt them down to prove his innocence.
It could also be a corrupted version of one of Fionna's friends. For example, if Cake were to be turned evil by some dark magic. Cake would know all of Fionna's weaknesses and could use that knowledge against Finn in his Fionna form, making her a formidable and tragic antagonist.
Princess Bubblegum could be a part of the fanfiction. She might be interested in the unique flora and fauna of the Canyon and could send Finn on a mission there. Marceline the Vampire Queen could also be added. Maybe she has some history with the Canyon and her presence could add an element of mystery and danger.
It could be a remnant of the Ice King's past enemies. Maybe one of the magical creatures that the Ice King used to annoy or fight against now sees Simon as a threat. Since Simon has the knowledge and power of the Ice King, this creature wants to stop him before he becomes too powerful again.
One plot idea could be that Finn is framed for stealing the Ice King's crown. He then has to go on a journey to find the real thief to clear his name. He teams up with Jake, and they start by investigating the last place the crown was seen.
It could be an old enemy of Finn and Jake who has come back to take revenge on their kids. Maybe the Ice King's descendants or something related to him.
Maybe it could be the Duke of Nuts. He might be jealous of Breezy's popularity or her special abilities. So he tries to frame her for something she didn't do in the Candy Kingdom, causing chaos for her and making her have to prove her innocence.
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.
Finn is the adventurous and somewhat naive human. His perspective is always interesting as he views the magical world of Ooo through the eyes of an outsider. The lady, whoever she is, usually has her own set of values and goals. If it's Flame Princess, for example, she has her power and her own kingdom to think about. Their conversations in the fanfiction are shaped by these aspects of their characters.
Obviously, Finn and the Jungle Princess are the main characters. Finn, a brave and adventurous boy, and the Jungle Princess, who has her own set of skills and knowledge about the jungle environment.
The main characters are Finn, a brave and adventurous human boy, and the Water Princess. Finn is known for his heroics and his sword - fighting skills. The Water Princess has control over water and is from the Water Kingdom.