Perhaps Percy raised in Atlantis could be involved in a mystery related to the ancient history of Atlantis. There could be a hidden power or artifact that he has to find before the wrong people do. As he searches, he uncovers secrets about his own family that are tied to Atlantis. Along the way, he has to face challenges like underwater monsters that are specific to the Atlantean depths and overcome them using his wits and powers.
Maybe Percy is sent on a quest to Atlantis because there are rumors of a traitor among the Atlanteans who plans to side with the Titans. Percy has to navigate the complex political situation in Atlantis and figure out who the traitor is before it's too late. He also has to deal with the Atlantean magic which is different from what he's used to.
In fanfiction where Percy is raised in Atlantis, his powers could evolve in unique ways. The energy or magic of Atlantis might enhance his water - related abilities. He could gain the ability to communicate with sea creatures in a more profound way, or perhaps he could control not just water but also other elements related to the underwater environment like the currents and the temperature of the water more precisely.
One idea could be that Percy has unique powers given to him by Chaos. For example, he might be able to control the very fabric of the universe in small ways. Maybe he can manipulate time on a very limited scale, like slowing it down during a fight to gain an advantage.
One idea could be that Percy is raised by Poseidon. He grows up with a deep understanding of the sea and its powers from an early age. This might lead to him having a more solitary childhood as he has to hide his connection to the god. Another plot could involve Percy being raised by Athena. He would then be extremely intelligent and strategic, perhaps getting into trouble at school for being too smart. A third idea is if he's raised by Apollo. He could have a natural talent for music and healing, and his adventures might revolve around using these gifts in a world that is both dangerous and full of mythical creatures.
A possible plot could involve Percy being sent to Atlantis on a mission by the gods. In Atlantis, he uncovers a conspiracy among some of the Atlantean nobles who are planning to overthrow the current ruling family and take over the power. Percy then has to decide whether to get involved in the internal politics of Atlantis or just focus on his original mission. Along the way, he also learns about the unique culture and magic of Atlantis, which is different from what he's used to in the surface world.
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.
A possible plot could involve Percy being raised by Poseidon in a hidden underwater realm. He would be trained in the ways of the sea gods from a young age. This could include learning to communicate with all sea creatures, not just the ones in Greek mythology. He might have to face challenges like underwater mazes or taming a powerful sea monster to prove his worth as Poseidon's son. As he grows, he could start to feel a longing for the surface world and the mortals there, leading to a conflict between his loyalty to his father and his curiosity about the world above.
One idea could be Percy having to face a new underwater threat that is specifically targeting Atlantis. Maybe a powerful sea monster that was once imprisoned by the Atlanteans has been released and Percy has to figure out a way to stop it without causing too much destruction to the underwater city.
One plot idea could be Percy having to deal with political intrigue in Atlantis. Since he's the king, different factions might try to influence him or even overthrow him. Another could be Percy using his powers to explore uncharted areas of Atlantis and discover ancient secrets. Maybe he finds a powerful artifact that has the potential to either save or destroy Atlantis.
One idea could be exploring the backstories of the secondary characters more in - depth. For example, what was the life of the mechanic before joining the Atlantis expedition? Another could be a story where the Atlanteans start to rebuild their civilization above ground, facing new challenges. Maybe a third idea could be a time - travel fanfic where modern characters go back to the prime of Atlantis and see how their technology and culture were at their peak.