There could also be a demigod who has been corrupted by the power of the God of Death. This demigod might have made a deal with the god in exchange for power and now is trying to undermine Percy and his friends to ensure the God of Death's success.
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.
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.
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.
The Titans could still be antagonists. Even though they were mostly defeated in the original series, some of them might have found a way to use the power of the night to their advantage. For example, Kronos could have some remnant of his power that is now tied to the night and he wants to use Percy, as the God of Night, to regain his full strength.
The main antagonists could be the Titans who are still bitter about their defeat and want to disrupt the new balance that Percy is trying to create. They might see Percy as a threat to their plans of regaining power. Another group could be some rogue demigods who have been corrupted by power and don't want Percy to enforce balance as it would limit their own abilities to do as they please.
The Titans could be the main antagonists. They might see Percy as a threat to their plans of regaining power. Since Percy has the power of the stars, they could try to steal his powers or stop him from using them to protect the world.
The Titans related to chaos and destruction of nature could be great antagonists. For example, Cronus might want to use Percy's newfound power as the God of Nature to further his own dark plans.