Perhaps a group of rogue demigods who believe that Percy being the first - born of Poseidon gives him too much power and influence. They form a secret society to try and bring him down, using their own unique skills and powers against him.
A powerful Titan who has a grudge against Poseidon could be a great antagonist. This Titan might be trying to undermine Poseidon's power by targeting his first - born, Percy.
A powerful Titan who sees Percy as a threat to his plans. Since Percy has the power of Zeus and Poseidon, he could potentially stop any chaos the Titan wants to create. The Titan might try to turn Percy's friends against him or find a way to strip him of his powers.
A powerful water nymph could be the main antagonist. Maybe she's jealous of the special bond between Percy and Poseidon and the power of the lemon. So she tries to steal the lemon and use its power to take over Poseidon's domain.
Perhaps it could be a new evil force created as a result of Percy's sudden god - born existence. His new - found power could have unbalanced some ancient magic or prophecy, and this new evil entity rises to oppose him and restore the old balance. It might be a being made of dark magic that has a vendetta against Percy for disrupting the natural order of things.
Another plot could be Percy being sent on a quest by Poseidon to retrieve a powerful and ancient artifact that was stolen from the sea god's domain. Along the way, he meets other demigods who have their own motives regarding the artifact, and he has to figure out who to trust.
It could be an ancient and powerful Titan that has risen again. This Titan might have a vendetta against Poseidon and targets Percy as a way to hurt him. Since Percy is Poseidon's son, his death in Poseidon's arms would be a great blow to the sea god.
In addition to them, other characters from the Percy Jackson series might also make appearances. For example, Grover could be involved as Percy's friend. However, the core of the fanfiction usually revolves around Sally, Poseidon and Percy. Their relationships, like Sally's love for Percy and Poseidon's fatherly connection (or lack thereof in some interpretations), are what drive the story. These characters are often developed further in fanfiction, showing different sides of them compared to the original books.
The allure lies in the fact that Percy already has a very interesting backstory. As the Champion of Poseidon, fanfiction writers can expand on his powers and his place in the mythological world. For instance, he could be sent on quests that are specifically related to Poseidon's domain, like protecting a sacred sea temple from an ancient evil.
In fanfiction, their relationship could be developed by having Poseidon be more involved in Percy's daily life. Instead of just popping in during big events, he could be there for the small things too. For instance, helping Percy with his schoolwork that involves marine biology. They could also have a project together like building a new safe haven for sea nymphs. Through this, they would bond over creating something positive. Also, Percy could start to take on some of Poseidon's responsibilities in the ocean, with Poseidon guiding him. This would not only strengthen their bond but also show Percy growing into his role as Poseidon's son.
Maybe there is an old, powerful being who sees Percy's wings as a threat to his rule. This antagonist would try to stop Percy from fully realizing the potential of his wings. For instance, he could set traps or send minions to attack Percy whenever he tries to fly to new places to explore his powers.
It could be a powerful demigod who has been corrupted by power. This demigod wants to be the sole champion of the arena and will stop at nothing to defeat Percy.