Another option is a corrupted god. There could be a god who has been influenced by dark forces and wants to use Naruto's power for his own evil purposes. He could pretend to befriend Naruto at first and then turn on him when the time is right.
The Titans could be the main antagonists. They might see Naruto as a threat to their plans of regaining power in Olympus. Since Naruto is trying to uphold the order in Olympus, the Titans would oppose him.
It could be an ancient demon lord who wants to enslave both the demon prince and Naruto. This demon lord might see the unique bond between them as a threat to his plans of domination.
A primordial being, like Chaos or Nyx, could also be the main antagonist. If they feel that the Olympians have become too powerful or are misusing their power, they might decide to wipe out the entire demigod race. Percy would then have to find a way to either convince them to spare the demigods or find a way to defeat them, which would be extremely difficult as they are among the most powerful entities in Greek mythology.
Another possibility is a new, unknown force from the Underworld. Maybe Hades has lost control over some dark entity that has grown in power and now aims to take over Olympus. This entity could be extremely powerful, with abilities that the demigods have never faced before. It would be a great antagonist as it would force the Percy Jackson characters to adapt and find new ways to fight in order to save Olympus.
Well, it could be about Naruto being somehow transported to the world of Greek mythology which is related to Olympus. Maybe he has to face challenges like the Greek heroes did, with his ninja skills. He might meet Greek gods and goddesses and form alliances or rivalries with them.
Well, it could be about Naruto being somehow transported to the world of Greek mythology in the Prince of Olympus setting. Maybe he has to learn the ways of the Olympians and use his ninja skills in a new context. For example, he might face off against mythical beasts using his shadow clone jutsu.
It could be someone from the Naruto universe who doesn't like Naruto's newfound fascination with Aku. For example, Danzo might see Naruto's interest in Aku as a threat to the stability of the village and try to stop Naruto from further exploring Aku - related things. So, Danzo could be the main antagonist in this case.
A new and powerful Titan who has been hiding and gathering strength all this time. He could be jealous of the Olympians' power and wants to take over. Percy would have to figure out his weaknesses and stop him before he causes too much destruction.
Maybe a new character could be introduced as the main antagonist. This character could be from a hidden village that has been secretly observing Naruto's growth. They realize his potential as a weapon and plan to steal his power or use him to start a war against other villages.
Krampus himself could be the main antagonist. Since he is a figure known for punishing the naughty, his actions in the Naruto world might be seen as unjust. He could be going after ninjas without really understanding their motives, causing a lot of trouble for the main characters.