Perhaps Percy is caught in a conflict between his loyalty to his mother Amphitrite and his friendship with surface - dwelling humans. There could be a situation where a powerful human organization is planning to exploit the resources of the sea in a harmful way. Percy has to find a way to protect the sea and its inhabitants without completely severing his ties with the human world. He could try to negotiate with the humans, using his knowledge of both worlds to come up with a sustainable solution.
A plotline could revolve around Percy discovering his unique powers as the son of Amphitrite. Maybe he has the ability to control not just water, but also the emotions of sea - dwelling beings. This could lead to a story where he has to use this power to calm a raging sea god who is about to destroy coastal cities in a fit of rage. He would have to navigate the complex politics of the underwater realm as well, as there could be factions that either support or oppose his new - found power.
Another plotline could involve Percy having to protect the concept of Order from a powerful antagonist who wants to spread chaos. This could lead to epic battles and Percy having to use his unique connection to Order to defeat the enemy. He might also have to gather allies along the way who believe in the importance of Order.
A plotline could involve Percy using his powers, which he may have inherited from Erebus, to fight against some great evil. Maybe there's a force threatening the world that only Percy, with his unique abilities, can stop. He could also be on a journey to discover more about his father and his own place in the world of the fanfiction.
He would likely be very strong in water - based powers. For instance, he could have the ability to create powerful whirlpools or tsunamis with ease. He might also have a natural affinity for sea creatures, being able to communicate with them effortlessly. This would make him a great protector of the underwater world.
One common theme is family. Since Percy is the son of Poseidon and Amphitrite is Poseidon's wife, it often explores how Percy fits into this new family structure. Another theme is power. Percy already has his own powers from Poseidon, and Amphitrite being a goddess also has great power, so how their powers interact is often explored.
One possible plot could be Percy discovering his true parentage as the son of Amphitrite. He might struggle with his identity, as he has always known Poseidon as his father. This could lead to a journey of self - discovery where he has to learn about Amphitrite's powers and how they are now a part of him.
Perhaps in the fanfiction, Percy is influenced by some dark magic or a curse that makes him betray the son of chaos. The son of chaos then has to decide whether to forgive Percy or seek revenge. The story could also explore how their friends and allies are affected by this betrayal and whether they side with Percy or the son of chaos.
One interesting plot could be Percy's adventures in the underwater realm as the son of Amphitrite. He might discover new sea creatures and magical underwater places. Another could be his interactions with other demigods from a different perspective, since his mother is Amphitrite.
Well, it could be about Percy having to adapt to his new divine family. Maybe he has to learn special water - related powers from Amphitrite that are different from what he already knows. He could also face challenges from other sea deities who are jealous of his new status.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). There are often many talented writers who create stories about Percy being the son of Poseidon and Amphitrite. Another option is FanFiction.net. Just use the search function and type in relevant keywords to find some great reads.
There's a fanfic called 'Amphitrite's Influence on Percy'. This one focuses on how Amphitrite, being a powerful goddess, starts to influence Percy in different ways. It shows Percy growing in strength and wisdom under her guidance, and there are some really exciting adventures that he embarks on as a result.