Well, in the 'Naruto relives his life fanfiction', Naruto could potentially change the relationships in the Hidden Leaf Village. He might be able to reach out to characters like Sasuke in a different way, since he already knows Sasuke's pain and struggles from his first life. This could lead to a more harmonious bond between them much earlier in the story.
In such a fanfiction, Naruto would have a huge advantage. He could start influencing the politics in the village right from the start. Maybe he would try to reform some of the old and unfair systems in the Hidden Leaf. Also, he could use his experience to better handle the Akatsuki threat. He might come up with new strategies to counter their plans, knowing what they are going to do in advance.
If Percy relives his life, he could completely rewrite the prophecy. He may not make the same mistakes that led him to fulfill the prophecy in the original way. Instead, he could find new ways to deal with the threats to the demigod world, like finding alternative solutions to the problems caused by the Titans and the Giants. He could also have a chance to explore more about his mother's side of the family, which was not much delved into in the original story.
Sasuke is definitely one. Since Naruto already knows Sasuke's path in his previous life, he could change Sasuke's fate. Also, Kakashi. Naruto might interact with Kakashi differently, sharing his future knowledge which could impact Kakashi's decisions.
One such story is 'Replay' by Ken Grimwood. The protagonist keeps reliving his life from high school, trying to make different choices each time.
He'd likely become more confident. Since he already knows what he's capable of, he can show it right away. For instance, in his first encounter with a monster, he won't be as scared and will be more assertive in using his powers.
One possibility is that Percy relives his life and focuses on training more intensively. He could learn new skills or magic earlier, which would change the course of his adventures. Maybe he would be more strategic in battles, and as a result, save more people and gain more respect in the demigod community.
Obviously, Harry Potter himself is the main character as the whole concept is about him reliving his life. His friends like Ron Weasley and Hermione Granger are also likely to be main characters as they are an important part of Harry's life in the original story and will surely play a role in this new scenario.
Obviously, Percy himself would be the main character. Annabeth would most likely be there too, as she is an important part of Percy's life. Grover would also be a likely character, being Percy's best friend.
He might try to change certain events. For example, he could prevent some of the tragedies that occurred in the original story. Maybe he would stop the Uchiha massacre by warning the clan or finding a way to resolve the conflict between the Uchiha and the village leadership.
If Naruto watches Naruto fanfiction, it would be a mind - boggling experience for him. He would see how his fans have taken the elements of his story and twisted them in different ways. He might come across fanfictions that explore his family history in more depth than the original series. Or he could find ones that pair him with characters in unexpected romantic relationships. He might even learn something new about himself from the fan interpretations, although he would also probably have to deal with a lot of out - there ideas that don't quite match his own understanding of his life.
If Naruto reads Naruto fanfiction, he'd probably have a mixed reaction. On one hand, he might enjoy seeing all the love fans have for his story and the world he lives in. He could laugh at the comical interpretations or be touched by the emotional ones. On the other hand, he might be a bit perturbed by some of the more inaccurate portrayals. But overall, it would be a unique experience for him to see how his story has been reimagined in the minds of his fans.