Monsters could also be antagonists. With Percy in a new, more powerful state as a godling, there could be new and more powerful monsters that are drawn to his power. These monsters might be stronger and more intelligent than the ones he faced as a demigod, and they could be after his godly essence or trying to use him for their own dark purposes.
It could be the navy. They see pirates as outlaws and would be determined to capture Percy and his crew. They might have better - equipped ships and more trained soldiers, making them a formidable foe for Percy and his fellow pirates.
A powerful sorcerer who wants to use Percy for his own dark purposes could be an antagonist. This sorcerer might see Percy's unique abilities as a key to unlocking some sort of forbidden magic.
The old enemies of Percy and Annabeth like the Titans or some of the more vengeful monsters could be the main antagonists. They might see the unborn child as a threat or an opportunity to get back at Percy and Annabeth.
The Titans could still be antagonists. Even though Percy is raised by Chaos, the Titans might see him as a threat to their plans of overthrowing the Olympians. They could try to capture or kill him to prevent him from interfering.
The other gods who are jealous of Percy's new connection with Hades could be antagonists. For example, Zeus might see Percy as a threat now that he has Hades' blessing.
The old - fashioned and conservative gods who don't like the idea of a new, young god like Percy shaking up their established order could be antagonists. They might see him as a threat to their power and try to undermine him.
In this kind of fanfiction, the Titans could also be antagonists. They might want to use Percy's power over time to their advantage. Maybe they think they can rewrite history in a way that would bring them back to power. Kronos, in particular, if he were somehow still a threat, could be a major adversary. He could try to manipulate Percy into using his time powers in a way that would benefit the Titans, like going back and changing events that led to their downfall.
The main antagonists could be some of the more traditional and power - hungry gods. For example, Hera might see Percy as a threat to the order she has maintained for so long. She could try to undermine him at every turn, fearing that his new ideas and his relation to Zeus will disrupt her plans. Another antagonist could be Ares, who might be jealous of Percy's potential power and influence over the other gods and demigods.
The Titans who once fought against the gods could also be antagonists. They might view Percy's connection to the Primordials as a way to regain their power and oppose him at every turn. They could send their minions or use their own powers to try and stop Percy from fulfilling his role as the Champion.
The Titans could be great antagonists. They might see the Hunt as a threat to their plans for power and send their minions to disrupt it. Percy would have to fight off these minions, like the Telkhines, who are known for their metal - working skills and could create powerful weapons against the Hunt.