One idea could be Harry traveling back to the time of the founders of Hogwarts. He could interact with Godric Gryffindor, learn from him, and maybe even influence the way the houses are initially set up. Another plot could involve Harry going back to the time when Voldemort was a student at Hogwarts. He could try to understand what made Voldemort turn so evil and perhaps prevent some of the early events that led to his dark path.
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.
Another plot could be that they travel back to the time when the Chamber of Secrets was first opened. Ginny, having been possessed by Tom Riddle in her own time, could use her knowledge to stop the young Voldemort from causing so much harm. Harry could also help by using his skills and knowledge of magic from his own time to combat the basilisk and keep students safe.
Well, a plot could be that a character from the future, like a descendant of Harry Potter, travels back in time to the time of the Triwizard Tournament. This character could have some knowledge of future threats and tries to prevent certain events during the tournament that could lead to greater disasters in the future. They might try to stop Voldemort's minions from using the tournament as a cover to carry out their evil plans. They could also try to make sure that Cedric Diggory doesn't die, which would have a huge impact on the wizarding community as he was a very popular and promising young wizard.
Well, perhaps the time - traveling protagonist has a special ability that gets noticed by different female characters. Say, the ability to manipulate time in unique ways. Harry Potter characters like Cho Chang could be part of the harem. The time traveler could be from a future where the wizarding world is in chaos, and by traveling back, they try to change things. This draws the attention of various characters, who then form a harem around the time traveler as they all work towards a better future for the wizarding world.
A possible plot in the Harry Potter Teddy time travel fanfiction could be Teddy going back to the time of the founders of Hogwarts. He could be a sort of mediator between the four founders, especially if there were some disputes or misunderstandings that led to future problems in Hogwarts. Teddy could use his knowledge from the future to try and smooth things over. This would not only change the history of Hogwarts but also have a big impact on the future wizarding world he comes from.
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 and Ginny travel back in time to the Marauders' era. They could interact with James, Sirius, Remus and Peter, and perhaps change some small events that later have a big impact on the fight against Voldemort. For example, they could give the Marauders some tips on how to better hide from Snape, which in turn could make Snape less bitter in the future.
A possible plot is that a young wizard, who is a huge fan of Harry Potter stories in his own time, somehow gets transported back to Tom Riddle's era. He initially just wants to see the famous Hogwarts of that time. But then he crosses paths with Tom Riddle. He decides to use his knowledge of the future to try and befriend Riddle and change his path. However, he soon realizes that Riddle is far more complex and dangerous than he expected, and his actions start to have unforeseen consequences on the future.
Imagine Harry travels back to the time of the Triwizard Tournament. With the Elder Wand, he could expose Barty Crouch Jr. much earlier. This would prevent Cedric's death and change the outcome of the tournament. It would also lead to a different dynamic in the wizarding world as Voldemort's plan to return would be thwarted much sooner.