In this fanfiction scenario, Harry might find that time travel has unforeseen consequences. When he goes back to save his parents, he could accidentally change the magical world in ways he didn't expect. For instance, by saving his parents, some spells or magical artifacts that were created as a result of their death might not exist. He may also have to deal with the fact that his presence in the past could disrupt the natural flow of time. However, if he manages to keep his parents alive, he might grow up in a completely different environment, with a family that loves and protects him, and he could become an even more powerful wizard without the trauma of his orphanhood.
Well, perhaps Harry uses a time - turner that has been magically enhanced to travel far enough back. He could disguise himself and befriend his parents, subtly guiding them to safety. Or he could change the circumstances of their meeting, for example, if he makes sure that they meet in a more secure place. Also, he could try to influence the actions of the Marauders so that they are more vigilant in protecting his parents.
Maybe Harry's parents are alive but they've been working undercover for a long time against a different dark force that is allied with Voldemort. When they come back into Harry's life, they bring with them a whole new set of problems and mysteries. For example, they might have made some dangerous pacts with other magical beings in order to gain information. Harry has to deal with the consequences of these pacts while also trying to keep his family together. Another plot twist could be that they are alive but time - traveling is involved. They've been stuck in a different time period and Harry has to figure out how to bring them back to the present, all while Voldemort is still a threat.
In a fanfiction where Harry's parents are alive, it could be that they were secretly protected by a powerful ancient magic. They were transported to a different magical realm. When they return, they bring with them new magical knowledge and spells. They help Harry become an even more powerful wizard. For example, they teach him how to combine different types of magic in unique ways. They also work to improve the relations between the wizarding world and other magical creatures.
The whole story would be very different. Harry might not end up living with the Dursleys, so his personality could be less guarded and more confident. He may not have the same connection to the Sorcerer's Stone quest as he did in the original story. Also, the relationship between Harry and Snape would change completely since the reason for Snape's complex feelings towards Harry was his love for Harry's mother.
One idea could be that James and Lily Potter go into hiding with Harry instead of being killed. They could use powerful magic to disguise themselves and Harry, and then raise him in a secret location, teaching him magic from a very young age. This would change Harry's whole personality as he would grow up with the love and guidance of his parents.
The plot would change a great deal. For one, Harry wouldn't have to live with the Dursleys. He would grow up in a loving magical household. He'd be more confident and perhaps less of an underdog. His parents would be able to train him in magic from a young age, so he might be more advanced in his studies when he gets to Hogwarts. Also, the fight against Voldemort would be a family affair, with James and Lily Potter being key players in the battle against the Dark Lord.
A potential plot development might be that Voldemort, upon hearing that the Potters are alive, would be enraged and try to attack them again. This would lead to a new series of battles and challenges for Harry and his parents. They would have to work together to fight off Voldemort and his followers. Also, Harry's parents could help him unlock new magical abilities or knowledge that he didn't have access to before because they are more experienced wizards.
A fanfic with Harry's parents alive could have a plot where they discover a new threat to the wizarding world. Since they survived Voldemort's first attack, they are more vigilant. They raise Harry to be a strong wizard, teaching him both powerful spells and the values of the wizarding world. The story could involve them going on adventures together to find the source of this new threat. Along the way, Harry learns more about his family's history and his parents' experiences at Hogwarts. There would be a lot of family bonding and magical discoveries.
In some fanfics, a time - traveler might end up being the cause of Grindelwald's rise instead of stopping it. For example, in 'The Unintended Consequence', the main character's actions in the past accidentally give Grindelwald more power.
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.