Another plot could involve them traveling to a time just before the prophecy about Harry was made. They could try to stop the prophecy from being made in the first place. This would be a complex task as it involves a lot of magical elements and the interference of fate. But if they succeed, it would completely change the course of the wizarding world's history as they know it.
Perhaps they travel back to a time in Hogwarts' history. Harry could interact with his parents as a student, and Narcissa could see a different side of the school. They might end up changing the way certain relationships develop. For instance, Harry could influence his father to be less of a bully, which in turn could have a huge impact on the future friendships and enmities at Hogwarts.
One idea could be that Harry travels back in time and tries to prevent Voldemort's first rise to power. He could try to stop Voldemort from creating his first Horcrux, thus changing the entire course of the wizarding war. Another plot could be Voldemort traveling to the future to see how Harry defeated him, and then using that knowledge to change his past strategies. Maybe he would try to find a way to destroy the prophecy before it was made.
A possible plot is that Harry and Tonks time travel to a point where the creation of Horcruxes was just beginning. They could try to stop Voldemort from splitting his soul, which would change the whole course of the wizarding world's history. They would face many challenges, like not being discovered by Voldemort or his followers. Also, Tonks might use her skills to distract or mislead some of the Death Eaters while Harry looks for a way to destroy the Horcrux - making materials or the places where Voldemort hid them.
A great plot might be that Teddy time travels to a time when Harry was a child living with the Dursleys. Teddy could try to secretly protect Harry from some of the bullying and neglect he endured. Or perhaps Harry and Teddy travel to a time long before Hogwarts was founded to learn about the origins of magic in the wizarding world, and they bring that knowledge back to their own time to help improve modern magic.
A more complex plot might be where both Harry and Tom are sent back in time to a point where the Hogwarts founders are still alive. They get involved in the founding events of Hogwarts, and Harry has to stop Tom from influencing the founders in a way that would make Hogwarts more exclusive and dark - sided. Harry could try to work with the founders to see the potential danger in Tom even at a young age.
Another plot idea is that Harry Peverell time - travels to different points in his own family's history. He gets to meet his ancestors, learn about their secrets and magic, and discovers that his family has a much larger role in the wizarding world's history than he ever thought. He could then use this knowledge to better understand his own place in the wizarding world and his connection to the Deathly Hallows.
They could time - travel to a time in Hogwarts' history when a great mystery was unsolved. Harry and Teddy could work together to figure out what really happened. For example, they might explore the mystery of the Chamber of Secrets long before Harry's own encounter with it. Teddy's inquisitive nature and Harry's experience could lead them to some amazing discoveries.
One idea could be that Harry and Fleur travel back to the time of the founders of Hogwarts. They could witness the creation of the houses and perhaps even influence some of the early magical traditions. For example, they might help Gryffindor and Slytherin better understand each other, which could have a huge impact on the future of Hogwarts and the wizarding world.
One idea could be that Harry discovers Narcissa has a secret mission to undermine Voldemort from within. Maybe she's been secretly helping the Order of the Phoenix all along and Harry stumbles upon this knowledge.
One idea could be a character going back in time to prevent a major dark wizard event that was overlooked in the original series. For example, they might try to stop the initial rise of Voldemort before he became so powerful.
Well, perhaps Harry travels back in time to the very beginning of the wizarding world. He discovers secrets about magic that have been long forgotten. He then uses this knowledge to make the wizarding world more inclusive and less divided by blood purity. Or he could time - travel during his own time period, like going back to his first year at Hogwarts over and over again to change small things that have big consequences in the future, like making sure Neville is more confident from the start.