The main antagonist could be a sea witch who has a vendetta against Percy and decides to use the aquarium as her lair to plot against him. She could be causing trouble for the aquarium animals and staff, luring Percy in to face her.
Maybe it's a rogue demigod who has gone bad. This demigod could be using the aquarium as a place to experiment with dark magic and is creating all sorts of problems. He might be trying to control the minds of the sea creatures in the aquarium for his own evil purposes. He would be a formidable opponent for Percy as he also has demigod powers but is using them for wrongdoings.
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.
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 main antagonists could be the Titans who are still resentful of the gods and want to disrupt Demeter's power. Since Demeter is in charge of fertility and the harvest, the Titans might try to cause chaos in that area. Another option could be some dark nature spirits that have been corrupted and oppose Demeter's rule over nature.
The Titans could be the main antagonists. Even though they have been defeated before, some of them might have found a way to return or their followers could be causing trouble. They see Artemis as a threat or a target for their revenge against the gods. Another possibility is a new group of powerful monsters that have banded together and are specifically targeting Artemis for her power and influence among the demigods.
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.
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 Roman gods could be antagonists. Since Percy is a Greek demigod, they might not fully trust him as the King of Rome. They could try to undermine his rule, believing that he will favor the Greek side in any disputes or decisions regarding the demigod world.
Some of the more powerful demons in Hell itself could oppose Percy. They might be jealous of his sudden rise to power as the Prince of Hell. These demons could have their own agendas and territories in Hell that they don't want Percy interfering with. They could use their dark magic and legions of lesser demons to try and get rid of him.
Gaea herself would obviously be a main antagonist. She has the power to manipulate the earth and all its forces against Percy. Her children who are fully committed to her cause could also be major antagonists. They might have powers related to the earth and be very difficult for Percy to defeat.
The Titans could still be a major threat. They might see the Beast King as a new power to either control or destroy, and they would oppose Percy's attempts to make peace with the beast kingdom.