Maybe there is an old, powerful being who sees Percy's wings as a threat to his rule. This antagonist would try to stop Percy from fully realizing the potential of his wings. For instance, he could set traps or send minions to attack Percy whenever he tries to fly to new places to explore his powers.
There could be a group of people who believe that Percy's wings are a sign of evil. They would be the main antagonists, constantly chasing Percy and trying to 'purify' him, which really means trying to get rid of his wings and his special abilities. They might be a fanatical religious group in the world of the fanfiction who have their own set of beliefs that are in direct conflict with Percy's existence as a winged being.
Another option is a group of anti - magic fanatics. They view Percy's wings as an abomination because they are a result of magic. They hunt Percy down, believing that by getting rid of him and his wings, they are protecting the world from the 'evils' of magic.
It could be a jealous demigod who sees Percy's wings as an unfair advantage. This character might try to find a way to cut off Percy's wings or steal the power that gave him the wings.
A powerful Titan who has a grudge against Poseidon could be a great antagonist. This Titan might be trying to undermine Poseidon's power by targeting his first - born, Percy.
Maybe Percy uses his wings to fly to hidden places, like a secret island where ancient secrets are hidden.
Perhaps it could be a new evil force created as a result of Percy's sudden god - born existence. His new - found power could have unbalanced some ancient magic or prophecy, and this new evil entity rises to oppose him and restore the old balance. It might be a being made of dark magic that has a vendetta against Percy for disrupting the natural order of things.
It could be a power - hungry dragon from a minor tribe that feels overlooked. This dragon might see Peacemaker as an obstacle to their own plans of taking over and ruling all the tribes. They would use devious means, like creating false alliances and then betraying them, all while trying to get rid of Peacemaker. They might also try to turn Peacemaker's friends against him by using their insecurities and secrets. In this way, they would be a formidable antagonist in the fanfiction.
It could still be Orochimaru. His plans for immortality and power - seeking don't really depend on Naruto's birth time. So he would still be a major threat to the village and Naruto himself.
It could be a jealous demigod. Maybe someone who has always been envious of Percy's popularity and abilities. Now that Percy has wings, this demigod sees it as an opportunity to make Percy look bad. They spread rumors that Percy's wings are a sign of him being cursed by the gods and try to turn other demigods against him while he's already feeling sick and struggling with his new wings.
It could be the warden of the prison. The warden might be a cruel and power - hungry person who wants to keep Percy imprisoned for his own reasons, like getting some sort of reward or power from those who put Percy there.
A powerful dark sorcerer could be the main antagonist. He might be seeking to use Percy's skills for his own evil purposes, and when Percy refuses, he becomes the target. The sorcerer would have a vast array of dark magic at his disposal, making him a formidable foe.