The traditional Titans could be antagonists. Since Percy has only half - Titan blood, they might see him as a threat or a tool to use against the gods. Kronos, if he were to somehow return or have a remnant of his power, would be a powerful foe. He could try to manipulate Percy's Titan side to turn him against the gods.
Another possibility is a new group of powerful beings created from the chaos of Percy's existence. His half - god half - Titan nature could have disrupted some ancient balance, and from that imbalance, a new threat emerges. These beings could be a mix of dark magic and primordial energy, and they target Percy because he is the source of the disruption.
One idea could be that Percy has to hide his Titan heritage from the other demigods at Camp Half - Blood. He discovers new powers related to his Titan side that are different from his godly powers. For example, he might have the ability to control certain aspects of nature in a more primal way compared to his Poseidon - related powers.
Obviously, the God of Death could be a main antagonist if he has some malicious plan. He might be trying to tip the balance of life and death in his favor, causing chaos in the mortal and demigod worlds.
The Titans could be great antagonists. They might see Percy as a threat to their power if he becomes the God of Elements. Kronos, for example, could try to steal Percy's newfound elemental powers to regain his full strength and take over the world again.
Another possible antagonist could be some of the more power - hungry Titans. They might see Percy as a rival or someone who could disrupt their plans. They could try to manipulate him or turn him to their side, and if he refuses, they would become his enemies. Also, there could be some ancient monsters that sense Percy's new power and want to destroy him before he becomes too powerful.
The obvious antagonist could be the Titan of Earth itself. Since it's a powerful being, it could be causing chaos and destruction as it tries to regain its former power and influence. Its very existence might be a threat to the balance of the world that Percy and his friends are trying to protect.
It could be a rogue god who sees the half - robot as a threat to the natural order. They might try to destroy it because they don't like the idea of a half - human, half - machine being in the world of demigods. Another possibility is a powerful Titan who was awakened and wants to use the half - robot's technology for their own evil purposes. Maybe the half - robot has some sort of power source that the Titan desires.
The Titans could still be antagonists. In the Percy Jackson universe, they are powerful and malevolent. A princess in a fanfic might have to face their wrath as they try to overthrow the gods and take over the world. They would see the princess as a threat, especially if she is allied with the gods or has some special power that could stop them.
Maybe some of the more malevolent Greek gods could be antagonists. If Naruto's presence disrupts the balance of power among the gods, some of the jealous or power - hungry ones could try to get rid of him. For example, Hades might see Naruto as a threat to his domain and try to stop him from interfering in the affairs of the underworld.
Perhaps some of the older, more traditional gods could be antagonists. They might see Percy as a threat because he's now the God of Dinosaurs and they fear the power he could gain from controlling these ancient and powerful creatures. For example, Zeus might be worried that Percy's new role could disrupt the existing power balance among the gods.
The Titans could be the main antagonists. They might want to use the powerful weapons Percy is associated with to regain their power and overthrow the Olympians.