A mermaid or merman with a special ability. They could be an ally or a foe to Percy. Maybe they have the power to control certain types of sea currents, and their motives are not clear at first. Another interesting character could be an old sea wizard who has been living in an underwater cave for centuries and has a lot of knowledge about the ancient powers of Poseidon.
One idea could be Percy having to face a new and powerful sea monster that is awakened by an ancient curse. He'd have to use his powers in unique ways, maybe even discovering new abilities as the son of Poseidon. Another could be Percy traveling to a long - lost underwater civilization that has a connection to his heritage as an ancient son of Poseidon.
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.
One possible plot could be Percy struggling with the duality of his parentage. He might face discrimination from both the children of Hades and Poseidon. For example, the children of Poseidon could see him as a threat because of his connection to Hades, and vice versa. He could then go on a journey to prove that his dual heritage doesn't define him, but rather gives him unique powers to protect the world of the gods and mortals.
How about a character named Riven? He is a young boy who was abandoned near a river and was adopted by the river spirits. He has the power to communicate with fish and other river creatures. Riven is initially very distrustful of Percy because he sees Percy as an outsider. But as they get to know each other, they become friends and Riven helps Percy understand some of the more hidden aspects of the rivers, like the places where the rivers' memories are stored.
A mermaid friend would be a great side character. The mermaid could help her understand the deeper parts of the ocean and give her advice on using her powers in new ways. For example, teaching her how to communicate with sea animals on a more profound level.
Maybe Percy discovers a prophecy that involves both his father Poseidon and Lupa. He has to journey to different realms under the sea and in the Roman wolf - related places. Along the way, he meets new demigods and creatures that either help or hinder him in fulfilling the prophecy.
One interesting plot idea could be Percy having to team up with a demigod child of Leto. They could go on a quest to find a long - lost artifact that has the power to restore balance between the sea and the moon's influence. Since Percy is the son of Poseidon, his control over water and the other demigod's connection to Leto's lunar powers would create a unique dynamic.
One interesting plot idea could be Percy having to deal with the conflicting natures of his 'parents'. Since Gaia represents the earth and Poseidon the sea, he might struggle with controlling both earth - related and sea - related powers. For example, when he's near a beach, he could accidentally cause small earthquakes or make strange things happen to the sea life. Another plot could involve Percy facing a threat that only his unique dual - heritage can defeat. Maybe an ancient evil that is both of the land and the sea, and Percy has to figure out how to combine his powers in a new way to stop it.
A great plot could be Percy getting into trouble in the mortal world and Poseidon having to come to his rescue without revealing his true identity to the mortals. This would create a lot of tension and interesting scenarios where Poseidon has to use his powers subtly. Also, they could be on a mission to save a group of sea creatures that are being hunted by an unknown and powerful force. Poseidon could be teaching Percy about the importance of all sea life along the way.
A cool plot could be Percy being sent on a quest that involves mending the rift between Zeus and Poseidon. Along the way, he meets old foes and new allies. He has to use his wits and the powers he got from his fathers to convince other gods and demigods to support his cause. For example, he might have to show the children of Zeus and Poseidon that they can work together instead of being rivals.