In this fanfiction, it could be some of the Titans who are still loyal to Kronos' old ways. They might see Percy as a tool to bring back Kronos' full power. They would try to manipulate Percy into joining their cause, and when he refuses, they become the main antagonists trying to either kill him or use his power against his will. Another possible antagonist could be a powerful demigod who is jealous of Percy's new - found status and wants to take it for himself.
The Olympian gods could be the main antagonists. Since Percy is the adopted son of Kronos, they might see him as a potential threat and try to stop him before he can fulfill any of Kronos' possible plans. They may send their demigod heroes after him.
The other Titans could be main antagonists. For example, Hyperion who is known for his great strength and his desire for destruction. He might not fully trust Percy and constantly try to undermine him in Kronos' eyes. Another could be Atlas, who is already a major foe in the Percy Jackson series. He might see Percy as a threat to his own plans within Kronos' regime.
Well, the main antagonists could be a group of rogue demigods who have formed a cult around Kronos. They believe that Percy, as the heir of Kronos, should lead them in overthrowing the current order of the demigod world. But Percy doesn't want any part of it, so they become his enemies. Another possible antagonist could be an ancient being that was once in league with Kronos and now sees Percy as a means to resurrect Kronos or continue his dark legacy. This being would stop at nothing to force Percy to embrace his role as the heir.
Well, in a 'Percy Jackson Seventh Son of Kronos' fanfiction, the plot could center around Percy being hunted by both the gods and the Titans. Since he's the seventh son of Kronos, he has a unique power that both sides want to either control or destroy. Percy has to go on a quest to find a way to neutralize this new aspect of his identity so he can live in peace. Along the way, he teams up with some unexpected allies, like a group of demigods who are also outcasts for various reasons.
It could be the Titans who see Percy as a threat. Since he is the son of Kronos, they might want to either use him for their own purposes or eliminate him to prevent any interference in their plans.
Some of Kronos' minions, like the monsters he commands, could be antagonists. For instance, the dracaenae. They are always eager to prove themselves to Kronos and might see Percy as competition within Kronos' ranks. They could constantly try to undermine Percy's position and cause trouble for him.
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 Titans could be great antagonists. They might see Percy as a threat to their plans of regaining power. Since he has the power of Aether, they could fear that he'll be able to stop them in ways other demigods can't. Another option could be some dark demigods who are jealous of Percy's connection to Aether and want to steal his powers for themselves.
The Titans could still be a major threat. For example, Kronos' followers who might be trying to rise again and regain power. They could target the children of Athena because of their wisdom and strategic abilities, seeing them as a big obstacle to their plans.
The Titans could be great antagonists. Since they often oppose the gods, they might see Percy as a threat, especially if he is the son of Leto. Kronos, for example, could try to stop Percy from fulfilling any prophecies related to Leto or the gods in general.