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.
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.
Yes, in some fanfictions, Percy Jackson can be depicted as a Titan. Fanfiction allows for a lot of creative liberties, and authors might reimagine Percy's character in this way for various reasons, like exploring new storylines or power dynamics.
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.
Perhaps in the fanfiction, the Titan of Earth has a connection to Percy's family heritage that he was unaware of. This discovery leads Percy on a journey to understand his own powers better. He has to face challenges like dealing with the Titan's minions who are trying to use Percy to fully revive their master. Along the way, he also has to balance his normal demigod life and relationships.
Another option could be a group of rogue demigods who have turned to the dark side. They follow Percy to Forks, hoping to find some powerful artifacts there. They don't care about the vampires or werewolves and are willing to cause destruction to get what they want. Percy has to stop them while also dealing with the local supernatural creatures.
The Giants might also be antagonists. They could band together and target Percy's kingdom, believing that by destroying him and his rule, they can undermine the power of the gods further. They might use their brute strength and strategic thinking to attack Percy from different fronts.
A likely main antagonist could be a powerful demigod who has turned to the dark side and wants to use chaos for his own evil purposes. He might see Percy as a threat to his plans.
In some fanfictions, it could be an ancient and powerful Titan that has been resurrected. This Titan might be seeking revenge on the demigods, especially Percy and Annabeth, because of their past victories over the Titans. Their powers and knowledge from the old times would pose a great threat to our heroes.
The main antagonists could be the Titans. They are powerful enemies in the Percy Jackson universe already. Now that Percy has Superman - like powers, they might see him as an even greater threat to their plans of overthrowing the gods. So, they would do everything in their power to stop him.
The main antagonists could be the remaining Titans who see Percy as the last obstacle in their plan to take over the world. Since Spartans were known for their great fighting skills, the Titans would want to eliminate Percy to ensure their victory.