Well, he might be able to change some of the bad things that happened. Maybe he could stop Umbridge from being so cruel. He could also be better prepared for the challenges that year brought, like the fight against Voldemort's forces.
His relationship with Sirius could be different. He might be more careful and not let Sirius get into danger so easily. Also, his relationship with Hermione and Ron could be strengthened even more as he would be more aware of how important their friendship is.
In 4th - year Harry Potter fanfics, a popular plot is Harry's training for the Triwizard tasks. He could be getting secret help from an unexpected source, like a former champion or a hidden teacher at Hogwarts. His relationship with Cedric Diggory can also be a major part of the story. It might start as a rivalry but develop into a mutual respect. Plus, the way the fanfic portrays the magic in the tasks, like the underwater scene in the second task, can be really interesting and unique to each writer.
Sirius Black can be a very popular character in 'harry potter 5th year fanfic'. Since he has a close relationship with Harry, fanfic writers might focus on his interactions with Harry during this year. Maybe he gives Harry some secret advice about dealing with the Ministry or the Death Eaters, which adds an extra layer of intrigue to the story.
His Defense Against the Dark Arts skills would be very useful. He could use spells like Stupefy or Protego to defend himself against any threats he might encounter, whether it's from dark wizards or regular WW2 dangers. His knowledge of potions could also come in handy. He might be able to brew some basic potions to heal the wounded or create something to help him blend in or hide. And his flying skills on a broomstick could give him an advantage in scouting areas or making a quick escape if needed.
One interesting plot could be about Harry forming an unexpected alliance with some Slytherin students during his 5th year. They might team up to uncover a mystery within Hogwarts that involves a dark magic item that was planted there by Death Eaters. This could lead to a lot of character development as they learn to trust each other despite their house differences.
One interesting plotline could be a deeper exploration of the Triwizard Tournament tasks from a different perspective. Maybe showing the behind - the - scenes preparations by the champions that we didn't see in the original book. Another could be a new relationship developing between characters, like a friendship between a Hogwarts student and a Durmstrang student that impacts the events of the tournament.
In many Harry Potter 8th year fanfic plotlines, the exploration of the wizarding world outside of Hogwarts becomes a major focus. Harry and his friends could be traveling to different wizarding communities, learning about their cultures and magic. They might encounter new magical creatures and have to figure out how to interact with them. There could also be a mystery involving a powerful, ancient magic that is somehow related to their 8th year at Hogwarts, and they have to solve it to keep the wizarding world safe.
Obviously, Harry Potter himself is often a main character. He is usually at the center of the story, still growing and learning in his 8th year. Ron Weasley and Hermione Granger are also mainstays. Their friendship with Harry continues to be important, and they are involved in whatever adventures or challenges come up. Ginny Weasley is another common main character, especially in relation to her relationship with Harry.
In Harry Potter 4th year fanfic, Viktor Krum is quite popular. He's the mysterious Durmstrang champion, and fanfic writers love to explore his character more deeply. They might write about his life in Durmstrang, his reasons for participating in the tournament, and his relationship with Hermione. Fleur Delacour is another popular character. Her beauty and grace, along with her Beauxbatons heritage, make her an interesting character to write about, especially in relation to the other champions.