One possibility for a Percy Jackson fanfiction centered around the Primordial Son of Chaos could be a story where Percy discovers a hidden prophecy related to this powerful being. Maybe he has to team up with some unlikely allies, like a group of minor gods who are also interested in the balance that the Primordial Son of Chaos might disrupt or uphold. This could lead to a series of adventures across different realms, including Tartarus, where they search for clues about the true nature of the Primordial Son of Chaos.
Another main character could be Chiron. He has always been an important figure in Percy's life. In this fanfiction, he might be the one who first discovers Percy's connection to Chaos and tries to guide him. Or he could be struggling to understand how to deal with Percy's new situation, especially if Percy's powers start to have unforeseen effects on Camp Half - Blood.
Perhaps some of the minor gods who are tired of the traditional power structure in Olympus could be Percy's allies. They might see his rebirth as a son of Chaos as an opportunity to shake things up. Also, any being that has a connection to the concept of change or new beginnings could potentially side with Percy. This includes spirits of innovation or even some forgotten heroes who sense that Percy is the key to a new era in the world of demigods and gods.
A possible plotline is Percy's struggle to control his chaotic powers. Since Chaos is such a powerful and unpredictable force, Percy may have to learn how to harness his abilities without losing himself. He could go on a quest to find a way to balance his powers, and during this journey, he discovers more about the true nature of Chaos and his place in this new world.
A possible plot could be that the Primordial Son of Chaos is slowly being awakened by the actions of the modern world. Percy senses this disturbance in the fabric of the cosmos. He then embarks on a journey to find out more about this entity. Along the way, he meets old foes and new friends. The story could involve a race against time as Percy tries to prevent the full awakening of the Primordial Son of Chaos if it's going to bring destruction, or he could be trying to ensure its proper awakening if it's meant to bring balance.
Well, in such a fanfiction, Percy might gain new and extremely powerful abilities as a son of Chaos. He could have control over primordial forces that are far beyond what he had as a son of Poseidon. For example, he might be able to manipulate the very fabric of reality in ways that would make him almost god - like.
Percy could interact with Annabeth in a really interesting way. Since he has the powers of Chaos and Order, Annabeth might be both intrigued and a bit worried. She could try to help him understand and control his powers better, using her wisdom. For example, she might research ancient texts to find out more about the nature of Chaos and Order and how Percy can co - exist with them.
One interesting plot could be Percy finding a way to balance the powers of Chaos and Order within himself. He might have to face challenges where the two opposing forces are at war inside him, and he has to learn to control them without being consumed. For example, when he's in a battle, the chaotic side might want to go all - out destruction while the order side wants a more strategic approach.
Obviously, Percy Jackson himself is a main character. He's the focus as the adopted son of Chaos. Chaos would also be a major character, as Percy's adoptive father. There might be appearances from old friends like Annabeth. She could be worried about Percy in his new situation or even try to help him. Another character could be some of the primordial deities related to Chaos. They might be either allies or antagonists to Percy depending on the story.
One interesting plotline could be Percy's struggle to adapt to his new role as the adopted son of Chaos. He might have to deal with new powers that are both a blessing and a curse. For example, he could have the power to manipulate the very fabric of reality, but it comes with the risk of losing his humanity. Another plotline could be Percy facing off against old enemies from his past in the Percy Jackson series, but now with his newfound powers and status. It would be a test of his loyalty and how much he has changed.