In this 'percy relives his life fanfic', it could be that Percy decides to explore different paths in his relationships. For example, he might try to be more than just friends with Annabeth earlier. He could also use his knowledge of the future to better understand the prophecies and find ways to fulfill them in a more peaceful and efficient way. This would not only change his own life but also the fate of those around him.
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, 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.
In 'harry relives his life fanfic', a possible plot point is that Harry gets to experience different magical adventures all over again. He could explore parts of Hogwarts he missed before or learn new spells earlier. Also, he might be able to influence the wizarding world's politics in a different way this time, like preventing some dark wizards from rising to power as he already knows their tricks.
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.
Well, in such a fanfic, perhaps Percy ends up in a new camp or place full of demigods he doesn't know. He has to prove himself without his mom's support. He could befriend some unlikely characters who help him understand more about his mother's disappearance. And in the end, he might find that his mom left to protect him from a greater threat that he now has to face on his own.
Maybe Harry realizes he made some mistakes in his past and decides to go back in time to correct them. He could try to save his loved ones who died in the original story.
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.
Maybe Percy leaves the hunt because he has some personal issues to deal with. For example, he might have a family matter that requires his full attention. Or perhaps he has a different mission or goal in mind that conflicts with the hunt.
One possibility could be that Percy's friends rally around him. They might take turns helping him with daily tasks like getting to class or bringing him food. His accident could also lead to some self - discovery, as he has to deal with being immobile for a while.
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.
In this fanfic, it might be that Percy is in a high - pressure situation like a really difficult quest. Things keep going wrong, his team isn't listening to his plans, and he just loses his temper out of frustration. He starts yelling at his teammates, accusing them of not taking things seriously enough.