One idea could be that Harry travels back to the time when Voldemort was still at Hogwarts as Tom Riddle. He could try to expose Tom's true nature early on, preventing all the future disasters. Another plot could be Harry going back to the time when his parents were killed and using his knowledge of magic to save them, and then seeking revenge on those who supported Voldemort at that time.
Well, perhaps Harry travels back to a crucial moment in the first wizarding war. He could team up with the Order of the Phoenix members from that time, but with his future knowledge. He might use this knowledge to outwit Death Eaters and foil their plans before they even start. And for revenge, he could ensure that those who caused great harm to his loved ones are punished in a more immediate and fitting way.
A possible plot is that Harry travels back to his own childhood at the Dursleys. He could use his magic in secret to make their lives a bit of a nightmare as revenge for all the years of mistreatment. But on a larger scale, he could also start preparing himself much earlier for the fight against Voldemort, learning spells and gathering allies that he didn't have in his original timeline. This way, he could change the course of events and make sure Voldemort is defeated much more easily and with less loss.
One idea could be that Harry comes back to Hogwarts as a teacher. He could pass on his knowledge and experiences to the new generation of students. Another plot could involve him returning to solve a new mystery that threatens the wizarding world, something that only he has the skills and knowledge to handle.
One idea could be Harry going back in time to his parents' era at Hogwarts. He could try to prevent certain events that led to their deaths. Another could be him going back to the time of the founders of Hogwarts and getting involved in the creation of the houses and Hogwarts' early rules.
One plot idea could be that Ichigo goes back in time to the moment before a major battle in the Soul Society arc. He uses his knowledge of the future to prevent certain casualties and change the course of the battle in a more favorable way for his side. Another idea is that he travels back to his own childhood and tries to understand his powers better from an earlier age, which could lead to him becoming even stronger in the present. Or perhaps he goes back to a time when the Hollows first started emerging in large numbers and tries to find the root cause of their sudden increase, which could potentially change the whole history of the Soul Reaper - Hollow conflict.
One plot idea could be Harry going back to the time when Voldemort was still at Hogwarts as Tom Riddle. He could try to prevent Riddle from turning into Voldemort by befriending him and showing him a different path. Another idea is that Harry goes back to the time of the founders of Hogwarts. He might learn ancient magic directly from them and use that knowledge to change the future of the wizarding world. Also, he could go back to a time when his parents were alive and get to know them better, which would be a very emotional journey for him.
One idea could be that Harry comes back through a powerful ancient magic that was dormant. Maybe a hidden spell in the Forbidden Forest is activated by his friends' strong desire to have him back. Another could be that Death itself gives Harry a second chance because he has unfinished business, like fully ridding the wizarding world of the last remnants of Voldemort's dark magic.
One idea could be that Harry goes back in time to prevent certain dark magic spells from being created. He might try to stop Voldemort's rise before it even starts by influencing the earlier events related to the Dark Lord. Another plot could be Harry going back and changing the way he interacted with certain characters like Snape, which could lead to a completely different relationship dynamic. Maybe he could also go back to save some characters who died in the original timeline, like Sirius Black.
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.
Perhaps a fanfic where Lily Potter's spirit is able to communicate with Harry Potter during his difficult times at Hogwarts. She gives him advice and guidance that helps him through his struggles, like dealing with Snape's complex feelings towards him, or facing the challenges of the Triwizard Tournament. This would add an emotional and magical element to Harry's journey.
A possible plot is that Harry travels back and accidentally disrupts the timeline. As a result, things start to change in the present he came from. He has to figure out how to fix it while dealing with Tom. Maybe he tries to mentor Tom in a positive way, showing him the value of Muggle - born wizards. This could create a ripple effect where Tom doesn't grow to hate Muggles and non - purebloods. Also, Harry could discover new spells or magic that was lost over time while in Tom's era and bring that knowledge back to his own time if he manages to return.
One possible fanfiction could be about Harry Potter accidentally stumbling upon a time - turning device in Hogwarts and ending up in the 1940s. There, he might meet the young versions of his professors, like Dumbledore, and get involved in the events that were shaping the wizarding world at that time, such as the rise of Grindelwald.