Percy's daily life would be different. Instead of going to Camp Half - Blood on land, he might attend an Atlantean school for demigods or those with special abilities. He would also have to adapt to the Atlantean diet which could be very different from what he's used to. And his friends would likely be a mix of Atlanteans and other demigods who have also made their way to Atlantis.
Living in Atlantis in fanfiction would completely transform Percy Jackson's life. For starters, he'd be in a new environment with different rules and social structures. The Atlantean culture might be more formal or have different values compared to what he's known on the surface. He would need to learn new ways of using his powers, as the water in Atlantis might have different properties. He could also meet new types of sea creatures that are only found in the Atlantis region, some of which could be friendly and help him, while others could be dangerous and pose a threat. Additionally, his relationships with his old friends on land might change as he becomes more immersed in the Atlantean way of life.
In this new role as king, Percy's relationship with the sea gods would become more complex. He's now in charge of a major part of their domain. Poseidon might have higher expectations of him and be more involved in his rule. Percy would also have to deal with other sea deities who might see his new position as a threat or an opportunity. His friendships with other demigods would be tested too. They might approach him with more respect but also expect more from him. Percy would need to balance all these relationships while also ruling Atlantis effectively.
His life would change drastically. He'd be living in a different environment, like the Batcave perhaps. He'd have to adapt to Batman's strict rules and training regime.
If Percy becomes a werewolf in fanfiction, his social life would be turned upside down. His friends at Camp Half - Blood might be hesitant to be around him, fearing his new, more unpredictable nature. He might be drawn to other werewolves or creatures of the night, which could introduce him to a whole new set of allies and enemies. Also, his training as a demigod would need to be adjusted to account for his werewolf abilities, like learning to fight in his new form without losing control.
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.
He'd probably have a different set of enemies. In Ancient Greece, the threats might be more centered around the local myths and power struggles. For example, he could be in the middle of a feud between two powerful city - states, each with their own godly patrons.
His life would change drastically. He would be a being of immense power. He would no longer have to fear the wrath of the Olympians as he would be on a similar or even higher level. He could shape the world as he wished.
His magical education could be enhanced. Sirius has a wealth of knowledge about the wizarding world, especially about things that are not taught in Hogwarts. He could introduce Harry to different magical creatures in a more hands - on way and teach him spells that are not part of the regular curriculum. Harry would also gain more confidence in his magical abilities as Sirius would constantly encourage him.
His adventures might be more limited by modern technology. For example, if he's being chased by a monster, he can't just disappear into the forest as easily as in the fictional world. There are cameras everywhere. But on the other hand, he could also use modern technology to his advantage. He could use a GPS to find his way to a quest location faster. Also, his battles with monsters would have to be more discreet to avoid causing panic among the normal people.
In fanfiction, Percy's move to San Francisco could bring new relationships. He could meet a girl who is also a demigod but has been hiding her identity. They start as friends but soon realize they have a special connection. Also, his relationship with his old friends might change. Some might visit him, and they have to adjust to the new environment together. He may also have new enemies. There could be a group of powerful witches in the city who see Percy as a threat to their plans of ruling the magical underworld of San Francisco.
His life would change drastically. He'd be exposed to different mythologies. Instead of just the wizarding lore, he'd learn about Greek gods and monsters. His combat skills might improve as he learns demigod - like fighting.