The Titans could still be antagonists. Since Percy has a connection to Hecate, they might see him as a threat to their plans of overthrowing the gods. They could try to capture him to use his powers for their own evil purposes.
Monsters with a particular hatred for magic could be the antagonists. They might sense Percy's strong magical aura as the son of Hecate and target him. For example, there could be a group of shadow - like monsters that are trying to extinguish all magic in the world, and Percy stands in their way.
The Titans could still be antagonists. They might see Percy as a threat in his new role as the champion of Hecate. Another could be some rogue demigods who are jealous of Percy's new power and want it for themselves. And there could be magical creatures that have been corrupted by dark magic and oppose Percy's efforts to bring balance with Hecate's magic.
It could be some of the pure - blood supremacists. They might see her as an abomination because of her connection to Hecate, who is not part of the traditional wizarding families. They would try to undermine her at every turn and make her life difficult at Hogwarts.
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.
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.
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 Titans could be great antagonists. They might see the daughter of Hecate as a threat to their plans of taking over. Her magic could be something they want to either control or destroy.
There could also be a group of demigods who are jealous of Percy's power due to his unique parentage. They might form an alliance against him, trying to steal his powers or at least make his life difficult. They could be led by a demigod who has a grudge against Percy or is power - hungry.
The obvious antagonists could be the angels. Since Percy is Lucifer's son, they would see him as a threat and try to stop him from causing any chaos or fulfilling any of Lucifer's supposed plans.
Obviously, the daughter of Hecate would be a main character. Percy Jackson himself would also be a key character. And maybe Annabeth could be involved too. She could be a bit skeptical of the daughter of Hecate at first because of her association with magic, which is often seen as unpredictable in the Percy Jackson world.
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.