Perhaps Harry travels back to the time when Voldemort was first rising to power. As the master of death, he tries to understand Voldemort's journey better. He could try to influence some of the events that led to Voldemort's creation, like preventing Tom Riddle from finding certain dark magic artifacts that enhanced his power. But he has to be careful not to disrupt the time continuum too much.
Another plot could involve Harry traveling to different points in time within the wizarding world's history. Since he's the master of death, he has a unique perspective. He could go back to the time of the founders of Hogwarts. He might learn secrets about the creation of the Deathly Hallows and how they are truly meant to be used, which then changes his understanding of his own role in the present.
One idea could be that Harry travels back in time to his parents' era. He uses his mastery of death to protect them from Voldemort. Maybe he can't directly interfere but finds a way to subtly change things, like giving his father a special charm that ends up saving their lives in a crucial moment.
One idea could be exploring how Harry uses his newfound status as the Master of Death to bring back some of the characters who were lost in the war, like Sirius or Lupin. He could then form a new kind of magical order with them to safeguard the wizarding world in a different way.
Well, in a time travel fanfic, perhaps Harry accidentally travels back to the time when the founders of Hogwarts were still alive. He might learn ancient magic directly from them and use that knowledge to defeat Voldemort in his own time. Or Ron could go back in time and end up changing some Quidditch matches in the past, which then has unexpected consequences for the present, like different teams winning championships and changing the overall Quidditch culture at Hogwarts.
One idea could be that Harry, as the Master of Death, uses his new - found power to travel through different magical realms to learn ancient magic. He could meet long - lost magical creatures and gain their knowledge. Another plot could involve him using his power to bring back some of the good characters who died in the war, like Sirius or Lupin, but with unforeseen consequences that he has to deal with.
Another idea is that Harry travels to a time when his family was being hunted by Voldemort but in a parallel timeline. He could try to find a way to merge the timelines in a way that saves his family in both timelines. This would involve a lot of complex magic and decision - making on Harry's part.
In a 'Harry Potter Dark Time Travel Fanfic', a character could travel back to the time of the first great wizarding war. They could attempt to save lives that were lost in that conflict, but in doing so, they unknowingly create new threats. For instance, saving a certain wizard might lead to that wizard being corrupted later and causing even more destruction.
Well, his mastery of death might give him some immunity to the normal rules of time travel. For example, he might not have to worry about the time - turner limitations. He could travel more freely through time.
One idea could be that the next - generation characters travel back to the time of the first wizarding war. They could accidentally change some small events, which then have big consequences on their present time. For example, they might prevent a certain spell from being cast that was crucial in the defeat of a dark wizard, and now they have to figure out how to fix it in their own time.
One idea could be that the new generation discovers a time - turning device left by Hermione. They travel back to the time of the first Triwizard Tournament. There, they meet a young Sirius Black and Remus Lupin, and get involved in the mystery surrounding Cedric Diggory's death in a new way.
A plot could also revolve around Harry going back in time to the origin of the Master of Death legend. He discovers secrets about how the power was first created and used. Along the way, he meets his ancestors or other powerful wizards from the past. This could change his understanding of his own power and his role in the wizarding world.
Well, perhaps in the fanfiction, the Arcobaleno are intrigued by Harry's status as the Master of Death. They might seek his help to break a curse that is related to death in their world. Harry, in turn, could learn from the Arcobaleno's skills and powers, leading to an exchange of knowledge and a more complex, intertwined plot.