One interesting plot could be that they go back in time to prevent a certain dark wizard from ever rising to power in the first place. They might use their knowledge of future events to stop key events that led to the wizard's ascension.
One interesting plot could be that Harry and Hermione travel back in time to change a major event at Hogwarts, like preventing the opening of the Chamber of Secrets. They might use their knowledge from the future to outwit Tom Riddle before he becomes Voldemort.
One interesting plot could be Hermione going back in time and preventing certain tragedies from the original story. For example, she might stop the death of a beloved character. Another could be her using her modern - day knowledge to change the course of wizarding history, like introducing new spells or concepts earlier.
One interesting plot could be Hermione and Sirius traveling back in time to prevent a certain dark wizard from rising to power. They could use their combined knowledge and skills to subtly change events. For example, Hermione's knowledge of magic history and Sirius' experience in the wizarding world could help them figure out how to stop the dark wizard's early supporters.
Another plot might involve Tom Riddle traveling to the future where Hermione exists. He could be intrigued by her magic, which is different from what he's used to. He might try to use her knowledge for his own gain in the past, but Hermione is too smart for him and outwits him at every turn. This creates a battle of wits across time.
One interesting plot could be that they use the Time Turner to prevent a major disaster at Hogwarts that was overlooked in the original story. For example, they might stop a dark wizard from infiltrating the school during an important event.
In some fanfictions, Harry goes back in time and decides to train himself better. He could seek out different magical knowledge or combat skills that he didn't have the first time around, which then changes his future interactions and battles. For example, he might learn ancient spells that give him an edge against Voldemort.
One interesting plot could be that Harry and Amelia travel back in time to the founding of Hogwarts. They get to meet the four founders and witness the creation of the school's magical protections. Another plot might be them traveling to a time when a great dark wizard was at the height of his power and they have to find a way to stop him without changing too much of the future.
One interesting plot could be Harry traveling back in time and meeting a younger Andromeda. He might end up changing some small events in her past that have a big impact on the future of the wizarding world. For example, he could prevent some minor tragedies in her family, which then alters the relationships between different pure - blood families.
Another plot might involve Voldemort traveling to the future to see the destruction he causes. This could lead to a moment of self - reflection for him. He might realize the futility of his actions and perhaps change his ways. It would be a fascinating exploration of his character.
One interesting plot could be Harry going back in time to prevent the death of his parents. He might use his knowledge of future events to outwit Voldemort before he becomes too powerful. Another could be Harry traveling back to his first year at Hogwarts and changing how he interacted with certain characters like Snape, which could lead to a different dynamic between them throughout the series.