One interesting plot could be that Harry, reborn in the past, uses his future knowledge to prevent certain tragedies from the original story. For example, he might stop Voldemort's rise much earlier by warning the right people or changing key events.
In some fanfictions, Harry reborn in the past might try to unite the wizarding world in a different way. Instead of the traditional Houses at Hogwarts causing some division, he could promote a more unified approach among students. He might also introduce new magical concepts he learned in his previous life, like advanced defensive spells, which could change the balance of power in the wizarding world.
Another plot could involve Harry being reborn as a different species in the wizarding world, say a werewolf. This would completely change his social standing and how he interacts with others. He would have to deal with the prejudice against werewolves while also trying to figure out his new life and his role in the fight against Voldemort.
One interesting plot could be Harry using his magical knowledge in the Pokemon world. For example, he might use spells to communicate with Pokemon in a unique way that normal trainers can't.
Perhaps Naruto's strong sense of justice and friendship would translate well in the Harry Potter universe. He could form unique bonds with the Hogwarts students, like teaching them some of his ninja values. He might also use his Rasengan technique in a creative way against dark wizards, adapting it to work with magic.
Another plot might involve Harry being summoned to the past right before a major event in wizarding history, like the creation of a certain dark object. He then tries to stop it from being made, which means he has to face some very powerful and dangerous wizards from that era. His presence in the past also causes some misunderstandings as people don't know who he really is and what his intentions are.
A possible plot is that Harry travels back to a time when the Marauders were in school. He could become friends with them and try to prevent some of the bad things that happened to them later. He could also use his knowledge of the future to help them in their pranks and adventures. This would be really interesting as he would be interacting with his father and his friends in a whole new way. It could also lead to changes in the future relationships and events at Hogwarts.
Well, you might find a plot where you're reborn into the Harry Potter fanfic as a muggle - born who has a unique ability. For example, you could have the power to communicate with magical creatures in a way no one else can. This would lead to all sorts of adventures, like helping Hagrid with some of his more dangerous or difficult creatures. Or perhaps you're reborn as a pure - blood who wants to break free from the traditional pure - blood elitism and fight for equality in the wizarding world. You could also be reborn as a character who discovers a hidden part of Hogwarts that no one else knows about and uncovers its secrets.
In some Harry Potter 8 fanfictions, there are plots where the characters continue to face new dark magic threats. For example, a new dark wizard might emerge, and Harry, Hermione and Ron have to use their combined skills and knowledge to defeat this new foe. Another plot could be about the exploration of the wizarding world's past in more depth, revealing secrets that were previously unknown.
One interesting plot could be Harry using his magical knowledge in the Naruto world. For example, he might use spells to enhance his ninja techniques, like using a spell to make his shuriken fly faster or more accurately. Another plot could be his interactions with the different ninja clans. He could bring new ideas to them based on his Hogwarts experiences.
In this type of fanfiction, a cool plot could be Harry traveling back to the time when the founders of Hogwarts were alive. He could learn ancient magic directly from them. He might also get involved in the politics of that time, helping to shape Hogwarts' early rules and traditions. This would not only change his own future but also the future of the entire wizarding world as he brings back forgotten knowledge and wisdom from the past.
Another plot might be Harry changing his relationship with Snape. He could go back and try to understand Snape better from the start, which might lead to a very different dynamic between them. This could potentially change how Snape behaves towards Harry and the other Gryffindors, and also how Harry views Snape throughout his time at Hogwarts.