Well, with Harry being three years older, perhaps he has already mastered some more advanced magic. In the fanfiction, this could lead to a different dynamic with his professors. For example, Snape might be more grudgingly respectful of his skills. Also, his relationship with Ginny could develop much sooner. They could be a power couple within Hogwarts, fighting side - by - side against the Death Eaters and helping to protect the school and the wizarding world in new and exciting ways.
In this fanfiction, a three - years - older Harry Potter might have a different experience with the Triwizard Tournament. Since he is older and more experienced, he could approach the tasks with a different strategy. He might also have different interactions with the other champions, Cedric Diggory and Fleur Delacour. Maybe he forms an earlier alliance with Cedric, and they work together to figure out the mysteries of the tournament. This could also change the outcome of the tournament and how the wizarding world perceives Harry.
Well, he could be more independent and decide to explore the Forbidden Forest on his own more often. He might discover new magical creatures that were previously unknown to Hogwarts students. This could lead to him getting into trouble with the teachers, but also make him a bit of a hero among his peers.
In some fanfictions, the older brother could be an outcast from the wizarding world due to a misunderstood magical ability. He then takes Harry under his wing and shows him a different side of magic, one that is not taught at Hogwarts. They might go on adventures together outside of the school, exploring ancient and forgotten magical places, and in the process, the older brother helps Harry become a more well - rounded wizard.
One plot idea could be Harry struggling to fit in at first in Slytherin because of the house's reputation for dark magic, but then he discovers the true values of loyalty and cunning within it. He could form an unlikely friendship with a pure - blood Slytherin who helps him see that not all in Slytherin are bad. Another idea is that Harry uses his newfound Slytherin traits to outwit Voldemort in a different way than in the original story, perhaps by using more stealth and manipulation.
Perhaps Harry is adopted by a Muggle family who are aware of the wizarding world. They could be a family of scientists who study magic from a scientific perspective. Harry would grow up with a unique understanding of magic, seeing it both as a wonder of the wizarding world and something that can be analyzed. This might make his time at Hogwarts very interesting as he tries to blend these two views of magic.
One possible fanfiction could be about Harry being 3 years older and already having a more advanced understanding of magic. He might start to question some of the Hogwarts traditions earlier and try to bring about changes. For example, he could be more involved in the politics of the wizarding world and use his fame to advocate for better treatment of house - elves.
One plot idea could be that Harry is framed by a powerful dark wizard who has infiltrated the Ministry of Magic. The wizard plants false evidence that makes it seem like Harry has committed a serious crime against the wizarding world. Harry then has to use his wits and the help of his true friends, like Hermione and Ron, to find a way to prove his innocence while being held in a magically fortified prison.
One plot idea could be that Harry starts having strange memories of Arthur's life. He might remember Arthur's relationships, like his bond with his family. This could lead to Harry trying to understand these memories and how they relate to his current life in the wizarding world. Another idea is that Harry discovers he has some of Arthur's skills, such as an affinity for working with magical devices like Arthur had with Muggle artifacts. It could create a whole new dynamic in his relationships with his friends and teachers as they try to figure out what's going on with him.
One idea could be Harry being raised by the Weasleys from a very young age. He'd grow up with their large family dynamic, learning magic in a more chaotic but loving environment. It might change his personality a bit, making him more outgoing and confident earlier. Another plot could be him being raised by Remus Lupin. Lupin would instill in him a deep understanding of the dark arts in a more academic way, and also teach him about acceptance of his own lycanthropy - like condition if he were to inherit it. And a third idea is Harry being raised by the centaurs in the Forbidden Forest. He'd learn their unique brand of magic, which is more in tune with nature, and have a different perspective on the wizarding world and its relationship with the natural world.
A plot idea could be that Harry's reincarnation as Salazar is part of a grand prophecy. As he uncovers this truth, he realizes that he has a greater role to play in the wizarding world. He might start to gather followers who believe in the old ways of Salazar. This could cause a division in the wizarding community, with some seeing Harry as a savior and others fearing that he will bring back the dark and exclusionary ways of Salazar. Harry would then have to decide how to use his new identity for the good of all wizards.
One idea could be that Harry, as Voldemort's soulmate, tries to use their connection to redeem Voldemort. He might show Voldemort memories of love and kindness in an attempt to change his dark ways.