A powerful Titan who was banished to the moon in the past and now wants revenge on the gods by destroying the moon could be the main antagonist.
Perhaps a group of dark sorcerers who believe that by controlling the moon's power, they can rule the world. They see Percy as a major obstacle in their plan and constantly try to defeat him.
A powerful Titan who has escaped from Tartarus could be the main antagonist. He might be seeking revenge on Artemis for her role in the previous battles against the Titans. This Titan could have new and terrifying powers that make him a formidable foe for Percy and Artemis.
A possible main antagonist could be a rogue demigod who was once rejected by the Hunt and now wants revenge. This demigod could have allied with some powerful monsters and is constantly causing trouble for the Hunt. For example, they might be stealing important artifacts or sabotaging the Hunt's missions.
It could be a powerful demigod who has been corrupted by greed. He wants to use the power of the earth for his own gain, like creating his own kingdom or something. So he's trying to stop Percy from protecting the earth because Percy stands in his way.
It could be an ancient and vengeful Titan who was once imprisoned by the Hunters' predecessors. Now free, it wants to destroy the Hunters to regain its lost power and take revenge on those who once locked it away.
It could be a rogue demigod who wants the throwing stars for their power. This demigod might have been shunned by the other demigods and sees the stars as a way to gain revenge and power over them.
A powerful and corrupted nymph could be the main antagonist. She was once a friend to the Hunt but was lured to the dark side by some ancient magic. Now she wants to destroy the Hunt and take over their territory. She uses her powers over nature to create all sorts of obstacles for the Hunt and Percy, if he's involved. She could also be very manipulative, turning some of the Hunt members against each other.
It could be a dark sorcerer who wants to steal the power of the moon for his own evil purposes. He sees Mune as the only obstacle in his way, so he tries to undermine Mune at every turn.
One possible plot could be Percy discovers he has a special connection to the moon. Maybe he's given the task of protecting a lunar artifact. Along the way, he teams up with some new demigod friends who also have moon - related powers. They face off against monsters who want to use the artifact's power for evil.
It could be a power - hungry Atlantean noble who wants to rule not just Atlantis but also the surface world. He sees Percy as a threat to his plans and tries to stop him at every turn.
A new and powerful demon that has emerged from the darkest corners of the underworld. This demon is bent on using Percy's connection to the overworld to break through the barriers between the two worlds and unleash chaos. It has powers that are different from what Percy has faced before, like the ability to manipulate the very essence of the underworld's environment.