It could be a new character introduced specifically for this fanfiction. Maybe a bully who has just transferred to Percy's school and takes an instant dislike to him. This bully might be jealous of Percy's popularity or has a grudge against him for no real reason other than wanting to assert dominance.
An old enemy of Percy's family could be the main antagonist. They might have a long - standing feud with Percy's family and see Percy as an easy target to get back at them. They could be a powerful and devious character who uses underhanded tactics to get what they want, including sending thugs to beat up Percy.
A corrupted authority figure in Percy's world could be the antagonist. For instance, a teacher or a local official who has been bribed or has some ulterior motive. They might order Percy to be beaten up to keep him quiet about something they are doing wrong, like embezzling funds or covering up a crime.
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 rival. This rival might have always been envious of Percy's popularity and skills. So, they manipulate Percy's friends into betraying him to get Percy out of the way and take over his position.
It could be a new character, like a powerful wizard who has a grudge against Percy for some reason. Maybe Percy accidentally disrupted one of his spells, and the wizard is out to get him, which makes Percy mad.
Sasuke could be one. Given their complex relationship, there could be a situation where Sasuke's newfound power or different ideology leads him to fight and beat Naruto in a fan - made story.
It could be a new Titan that has been awakened from an ancient slumber. This Titan might be angry at the gods for imprisoning them long ago and wants revenge. Percy's death could be part of the Titan's initial plan to strike fear into the hearts of the gods and demigods, but Percy's rebirth becomes a major obstacle for the Titan.
A group of bandits might be the main antagonists. They see Percy as someone they can easily rob or enslave since he is all alone. Percy would then have to use his skills to outwit them.
It could be a jealous demigod. Maybe someone who has always been envious of Percy's popularity and power. This demigod could spread lies about Percy that lead to his disownment, making them the main antagonist.
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.
It could still be Hawk Moth. Since Nathalie has been working with him, getting a Miraculous might make her a threat to his plans, so he could turn against her and become the main antagonist.