A powerful sorcerer could be the main antagonist. He might be jealous of Jack and May's emerging powers and see them as a threat to his own plans of ruling the land. So, he tries to stop them at every turn, creating various obstacles and sending his minions after them.
It could be an ancient evil spirit that has been awakened. This spirit wants to plunge the world into darkness and chaos. Jack and May stand in its way because they have a special connection to the light magic that can defeat it. The spirit uses its dark magic to manipulate people and situations against them.
Jack and May are the protagonists. In fanfiction, they could be given different personalities and backstories from the original source (if there is one). They might be best friends, siblings, or lovers, depending on how the author of the fanfiction chooses to portray them.
Another idea is that Jack and May are given a special task by an ancient being. They have to find a set of magical artifacts scattered across their land. However, there are other forces that also want these artifacts for evil purposes. Jack and May's journey is filled with challenges, and they also have to deal with their own personal growth and relationship development as they complete the task.
Another fanfiction idea could be a time - travel aspect. Maybe Jack and May get sent back in time to when their world was first created. They then have to figure out how to get back to their own time while also dealing with the primitive and often dangerous conditions of the past.
It could be an ancient ice - hating wizard. This wizard might have been banished long ago but now that Jack Frost has gone back in time, he sees it as an opportunity to stop winter from ever existing. He could use his dark magic to melt all the ice and snow in the past, which would have a ripple effect on the future.
In some cases, it could be a corrupted lost boy. Maybe one of the lost boys has turned bad due to jealousy or a desire for power. He sees the hurt girl as an easy target to assert his dominance or to gain something from her, like her friendship with Peter Pan. So he becomes the main source of trouble for the girl in the fanfiction.
It could be a powerful ninja who has made a deal with an otherworldly force to gain powers to match Naruto's. This antagonist might be jealous of Naruto's unique heritage and wants to prove that pure ninja power is superior. So, he uses dark magic or forbidden jutsu to become a formidable foe.
It could be a powerful ninja from another village who is jealous of Naruto's half - demon half - god powers. He might see Naruto as a threat to his own village's dominance and try to eliminate him.
It could be a powerful wizard who fears Harry's half - demon powers and wants to eliminate him. This wizard might see Harry as a threat to the wizarding world's order.
Maybe a rogue ninja who has been experimenting with dark magic. He discovers Naruto's unique nature and wants to steal his power. He believes that by combining Naruto's angel and demon fox powers, he can become invincible. So, he sets out to hunt Naruto down and tries every trick in the book to get his hands on that power.
The mermaids could be the main antagonists. They are known to be jealous and territorial in the Peter Pan stories. If the lost girl catches Peter Pan's attention in a way they don't like, they could make her life in Neverland miserable. They might use their alluring voices and their knowledge of the waters around Neverland to try and drive her away or lead her into danger.