Here are some plot ideas. The OC could be a researcher studying ancient sea civilizations. Poseidon, seeing this as an intrusion, decides to confront the OC but then realizes the OC's pure intentions and starts to cooperate with them. Or the OC is a shape - shifter who can turn into a sea creature and gets involved in a power struggle in Poseidon's underwater kingdom. Maybe the OC is a time - traveler who accidentally lands in Poseidon's era and has to adapt to the sea - god's rules while trying to find a way back to their own time.
One plot idea could be that the OC is a young mermaid who accidentally crosses into Poseidon's territory. Poseidon, initially angry, is intrigued by her courage. Another could be the OC is a mortal with a special power to control water to a small extent, and Poseidon takes an interest in mentoring the OC. Or perhaps the OC is a demigod sent on a quest by the gods, and Poseidon has to decide whether to help or hinder the OC.
Well, an interesting plot could be that the OC is a sailor who has a near - death experience at sea and is saved by Poseidon. In return, the OC has to perform a series of tasks for Poseidon. It could also be that the OC is an artist who dreams of painting the underwater world of Poseidon, and through some magical means, gets the chance. And yet another idea is that the OC is a lost soul who washes up on Poseidon's shore, and Poseidon decides to help the OC find their way back home, all while there are various underwater creatures causing trouble along the way.
One plot idea could be Percy having to go on a quest specifically guided by Poseidon's hidden messages in the sea currents. He has to decipher these messages to find a lost artifact that can save the underwater kingdom from a new, mysterious threat.
One idea could be that the OC is a mermaid and she accidentally saves Poseidon from a magical trap. Their relationship starts from there as Poseidon is intrigued by her kindness. Another plot could be the OC is a human who washes up on a secret island where Poseidon often visits. They gradually fall in love as she shows him the beauty of human emotions. A third plot could be that the OC is a goddess - in - training and Poseidon is assigned to be her mentor, but they end up falling for each other instead of just having a mentor - mentee relationship.
One idea could be Percy having to face a new threat underwater that only Poseidon can help him understand. Maybe a lost civilization that Poseidon once had a connection with is now causing trouble. Another could be Percy going on a quest to find a powerful artifact that was once Poseidon's but has been stolen, and Poseidon gives him special powers or guidance along the way.
Perhaps there's a story where Poseidon falls in love with Gabrielle. Xena has to protect her friend from the powerful god's advances while also trying to keep the balance in their world. This could lead to a lot of drama and action as they navigate the complex situation.
One plot idea could be a story where Poseidon and Athena have to team up to face a common threat to the pantheon. Maybe a new and powerful deity emerges who wants to overthrow the existing order. Their different powers and personalities would create interesting dynamics as they try to work together.
In this type of fanfiction, a possible plot is that Poseidon raises Percy away from Camp Half - Blood to protect him from the prophecy. As Percy grows up, he becomes more in tune with his water powers. He could then have adventures in the ocean, like finding lost civilizations or facing off against sea monsters that Poseidon sends his way to test him.
In a fanfiction, Poseidon and Zeus could be involved in a bet. The bet could be about who can better handle the affairs of a particular mortal city for a certain period. As they interfere in the mortal world, they learn more about each other's capabilities and develop a newfound respect.
One idea could be her going on a quest to find a lost trident piece that was stolen by a powerful new enemy. This would involve facing various mythical creatures and using her unique powers as Poseidon's daughter.
Another plot could involve Percy being ostracized by some of the other demigods because of his father. But then, he uses his unique connection to Poseidon to prove his worth. He might be able to call on sea creatures to help in a battle or to find something lost at sea. This would show that being raised by Poseidon gives him an edge.
One idea could be Harry using his Poseidon - given powers to control water in the Triwizard Tournament. For example, in the second task, instead of using gillyweed, he could manipulate the water to his advantage.