The Giants could also serve as antagonists. They are known for their strength and hatred towards the gods. The son of Hades, being related to the underworld, might be a target for them. They could try to capture him to use his powers to open portals to the underworld for their own evil purposes.
Another option could be some rogue demigods who are jealous of the son of Hades' powers. They might form a group and try to cause trouble for him and Percy Jackson. They could be working with some minor gods who have a grudge against Hades, creating a complex web of enemies.
Some of the more powerful and vengeful spirits in the Underworld itself could be antagonists. They might not want an outsider like Percy to be the Champion of Hades. They could try to undermine him at every turn, like spreading false rumors about him among the other Underworld denizens or directly attacking him when he's vulnerable during his quests in the Underworld.
Some of the more malevolent spirits or monsters from the Underworld could also be antagonists. They might resent Percy for being Hades' adopted son and try to undermine him. For example, a powerful undead general who believes he should be the one favored by Hades instead of Percy.
The Titans could be great antagonists. They might see Percy as a threat because of his connection to Thanatos and the power it could potentially give him. Also, some rogue underworld creatures, like powerful demons or vengeful spirits, could oppose Percy. They may not want Thanatos to have a more powerful influence through Percy.
The main antagonist could also be an ancient and powerful spirit from the underworld that was sealed away long ago. Percy's presence as the son of Hades accidentally awakens this spirit. The spirit then starts to spread darkness and despair throughout the world of the demigods, and Percy has to find a way to stop it before it's too late.
The Titans could be the main antagonists. Even though they were defeated before, they might see Percy as a threat because of his powerful parentage. They could try to stop him from fulfilling a prophecy or gaining more power. For example, Kronos might have some remaining influence and try to manipulate events to destroy Percy.
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.
It could be the Titans or some new creatures that are jealous of Percy's connection to Chaos. They might see Percy as a threat and try to stop him and Perlia from achieving their goals.