A character who was a villain in their past life but is reborn with a chance to change. They would struggle with their old instincts and the expectations of others, but could grow into a hero. This kind of character arc would be really engaging as they fight against their own nature and the prejudices of the demigod community.
It could be a new character, a half - blood who discovers they have the ability to shift into monsters. This character could have a troubled past, maybe they were abandoned as a child because of their strange power. They then have to learn to control it and find their place in the world of demigods.
Maybe one of the primordial deities, like Gaea, could be the main character. The fanfic could explore her thoughts and actions as she was one of the first beings. It could show how her relationship with other primordial deities influenced the future of the universe, and how her actions in the beginning set the path for events that would unfold much later in the Percy Jackson series.
The goddess Hera might also be a great antagonist. Since she has a history of interfering in the lives of demigods, she could be jealous or threatened by Percy's rebirth. She might try to stop him from reaching his full potential in his new life or cause trouble between him and his new allies.
A shy and overlooked demigod would be a great choice. This character could be someone who doesn't fit in with the more battle - ready or flashy cabins like Ares or Apollo. They find their place in Hestia's cabin and gradually become more confident as they embrace the values of the cabin.
It could be a new, powerful Titan that was previously unknown. This Titan might have a vendetta against Percy specifically, seeing him as a major threat to their plans. Maybe they manage to kill Percy, but his rebirth sets in motion a series of events that will lead to their ultimate downfall.
The main antagonist could be a powerful sorcerer who has found a way to enter the Percy Jackson game world. He has his own motives, perhaps he wants to steal the powers of the demigods for himself. He's able to manipulate the game's code to his advantage, creating illusions and false paths for Percy and his friends to follow. Or it could be an ancient spirit that was sealed away in the game but has been released. This spirit is angry at the gods and demigods and wants to destroy everything they hold dear. Also, a group of bandit demigods could be the main antagonists. They don't follow the rules of the game or the real - world demigod society and are causing chaos everywhere they go, trying to steal valuable items from both the game world and the real - world locations.
Nyx could be a great main antagonist. She's a primordial goddess of the night and Tartarus is her domain in a way. She might see Percy and his friends as intruders and try to stop them from leaving or achieving their goals. She could use her power over darkness and the unknown to create all sorts of obstacles for them.
It could be a new, powerful Titan that has emerged from the depths of Tartarus. This Titan might be seeking revenge on the demigods for imprisoning the other Titans and sees Hylla and Percy as the biggest threats.
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.
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.