Well, he might have a whole different set of skills. Maybe he'd be an expert at underwater magic that's unique to Atlantis. He could have friends among the Atlantean creatures and be more in tune with the ocean's secrets there than in the regular Percy Jackson stories.
His personality would likely be a blend of his natural Percy - ness and the influence of Atlantis. He could be more inquisitive about the history and magic of Atlantis. Since it's such a mysterious place, he'd constantly be seeking knowledge. At the same time, he might be a bit more detached from the surface world, having grown up in an underwater realm. His sense of humor might also change, adapting to the Atlantean way of life which could be more formal or refined in some ways.
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 of his main allies could be an Atlantean princess. She could be well - versed in the ways of Atlantis and help Percy navigate the complex social and political structures. Another ally might be an old Atlantean sage. The sage would have a vast knowledge of Atlantean magic and history, and could offer Percy valuable advice. There could also be a merman who is an outcast like Percy in some ways. They bond over their differences and help each other out in tough situations.
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.
It could be a power - hungry Atlantean noble who wants to rule not just Atlantis but also the surface world. He sees Percy as a threat to his plans and tries to stop him at every turn.
Obviously, Percy Jackson is one of the main characters. He is the son of Poseidon and has his own set of powers and a strong sense of justice. Another important character could be the current prince or princess of Atlantis. They might be Percy's ally or rival, depending on the story.
He might discover new sea creatures that are unique to Atlantis. Maybe he'll find a hidden school of magic there, or encounter ancient sea gods who have been forgotten by the outside world.
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.
One interesting plot could be Percy exploring the hidden, ancient parts of Atlantis that are full of forgotten magic and powerful artifacts. Maybe he discovers a long - lost prophecy that only he can fulfill, which involves uniting different sea - based tribes in Atlantis.