Emotionally, Harry would become more stable. He would have someone to talk to about his nightmares and fears. Sirius could comfort him and give him advice on how to deal with the traumas he has faced. They could also plan pranks together like James and Sirius used to do, which would add a sense of fun and normalcy to Harry's life.
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.
He might become more reserved. Without the constant use of magic and the wizarding world around him, he could be less confident. For example, he wouldn't be able to use magic to solve problems quickly, so he'd have to rely on his wits and Muggle knowledge more, which could make him a bit more cautious.
His friends would be different. Instead of Ron and Hermione, he might meet American - born wizards with different personalities and magical abilities. They could be more into modern American culture, like pop music and movies, and bring that into their magical adventures.
Harry's life in his animagus form would bring a whole new perspective to his fight against Voldemort. He could use his animal abilities for stealth and speed. As an animagus, he could get closer to Voldemort's lair without being noticed. He could also use his form to lead Voldemort's minions astray. His presence as an animagus could be a symbol of hope for the wizarding world in a different way. Instead of the boy - who - lived, he could be the mysterious animagus that always thwarts Voldemort's plans.
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.
Well, at first, Sirius would be in a state of shock and panic. But then he'd have to step up. Their relationship would shift from one of equals (in a sense, as Harry was growing up) to a caretaker - cared - for dynamic. Sirius would have to learn about Harry's likes and dislikes as a baby. He'd sing lullabies to him, comfort him when he cries. This new experience would make their bond stronger in a very different way. Harry might not remember all of it when he's turned back, but there would be a subconscious change in how he feels towards Sirius, a deeper sense of security and love that stems from those baby - days when Sirius was his whole world.
His life would be much quieter. No more noisy Gryffindor common room or bustling Diagon Alley. He could enjoy the peace of nature, listening to the wind in the trees and the calls of magical birds.
His life would be very different. For one, he'd have to deal with the extreme cold. Magic would be used in different ways, like creating warm shelters or finding food in the harsh winter. Also, he'd be far from his friends in Hogwarts, so he'd have to make new friends, maybe among the local magical folk or other outcasts who live in Alaska.
Harry's relationships might also change. Some students might be in awe of him and want to be his friend just because of his wings. However, others might be jealous. He would also have to deal with the stares and whispers. But overall, his wings could give him a new sense of confidence and independence, allowing him to face his enemies, like Voldemort, with a different kind of courage.
If Harry Potter becomes part of a Pokemon fanfiction, his life would change in many ways. Firstly, his daily routine would be different. Instead of going to magic classes, he'd be out in the wild looking for Pokemon. His friends might also change as he'd befriend other Pokemon trainers. His relationship with magic might even be affected. He could start using magic in new ways related to Pokemon, like using spells to communicate with them. And his sense of adventure would be redirected towards exploring the different regions in the Pokemon world.
His focus would shift from just magic and wizardry to also include the world of animations. He'd probably be constantly thinking about the next animation he wants to watch.