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.
A possible plot could involve a character going back in time to the beginning of the universe. This character, perhaps a demigod with special powers, witnesses the formation of the cosmos. They see how the different elements of the Percy Jackson world, like the Underworld, Mount Olympus, and the various magical creatures, came into existence. It could also explore how the concept of demigods was first conceived during these early times.
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.
The main villains could be the Titans. Since Percy has created a new universe, they might see it as an opportunity to rule over something new and powerful. They could try to overthrow Percy and take control of the universe - making process.
Annabeth could definitely be one of his allies. She's smart and resourceful, and her knowledge of architecture and strategy would be useful no matter where in the universe they are. She could help Percy figure out the layout of strange alien cities or fortresses.
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.
There could be an alien sidekick named Zorx. Zorx might be a small, furry creature with the ability to see in multiple spectrums, which helps Percy navigate through different planetary environments.
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.