The Titans could still be a major threat. Even though they were mostly defeated in the original series, some of their followers might see Percy's new role in Atlantis as a threat to their plans for chaos. They could try to undermine Percy's position in Atlantis and use the underwater city as a base for a new uprising.
The Atlantean nobles who oppose Percy's claim to the title of 'Prince of Atlantis' could be antagonists. They might be jealous of the power and influence Percy is starting to gain in Atlantis. They could plot against him, trying to frame him for crimes or start rumors to turn the Atlantean people against him.
Obviously, Percy Jackson is one of the main characters. He is the son of Poseidon and has his own set of powers and a strong sense of justice. Another important character could be the current prince or princess of Atlantis. They might be Percy's ally or rival, depending on the story.
The main villain could be a power - hungry Atlantean sorcerer who wants to use the power of Atlantis to rule both the underwater world and the surface. He might see Percy as a threat to his plans and try to eliminate him.
One idea could be Percy having to face a new underwater threat that is specifically targeting Atlantis. Maybe a powerful sea monster that was once imprisoned by the Atlanteans has been released and Percy has to figure out a way to stop it without causing too much destruction to the underwater city.
The Titans could still be antagonists. In the Percy Jackson universe, they are powerful and malevolent. A princess in a fanfic might have to face their wrath as they try to overthrow the gods and take over the world. They would see the princess as a threat, especially if she is allied with the gods or has some special power that could stop them.
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.
The obvious choice would be the Prince of Darkness himself. He could be a powerful being with the ability to manipulate shadows and dark magic, constantly trying to draw Percy to the dark side.
It's possible that the main antagonist is a new mythological creature that has the ability to manipulate minds. It has lured Percy and other demigods to the mental hospital and is slowly driving them insane in order to feed off their fear and chaos energy.
A group of hunters who have a vendetta against wolves could also be antagonists. They might be descendants of some ancient hunters who were cursed by a god to always hunt wolves. Percy, as the king of wolves, has to protect his pack from them.
The Titans could be great antagonists. They might see Percy as a threat to their power if he becomes the God of Elements. Kronos, for example, could try to steal Percy's newfound elemental powers to regain his full strength and take over the world again.
The Titans could be antagonists. They might see Percy as a threat since he's Nyx's champion. They could try to stop him from fulfilling Nyx's tasks or even try to turn him against Nyx.