A plot that I find very intriguing is when a character travels back in time to prevent the death of a loved one. For example, in some fanfics, Harry travels back to save Sirius. This creates a lot of tension as he has to deal with the time - turner rules and the possible consequences of changing the past.
There are also fanfics where Voldemort gets hold of a time - turning device. This leads to a completely different power dynamic as he tries to change history to his advantage. The author has to be really creative in how they handle this, and it often results in a very exciting and unpredictable plot. In some of these stories, the characters have to team up in new and unexpected ways to stop Voldemort's time - meddling.
Some fanfics might explore Nagini's connection to the other Horcruxes. Since she was the last Horcrux to be destroyed, perhaps there was some kind of communication or power exchange between her and the other Horcruxes that we didn't see in the books. This could add a whole new layer of mystery to the story.
Well, one intriguing plot is when Harry time travels and has to hide his true identity as a Gryffindor - turned - Slytherin. He has to act like he belongs there while secretly trying to figure out how to get back to his own time. Another plot could be that his time travel into Slytherin causes a rift in the timeline, and he has to fix it before everything goes wrong. For example, in some fanfictions, his presence in Slytherin changes the relationships between the houses and he has to make sure the good guys still win in the end.
One possible plot could be that Harry travels back in time and meets James' twin. Maybe they team up to change some events in the wizarding world. For example, they could try to prevent Voldemort's rise to power earlier on. This would create a whole new set of adventures and challenges for them.
The plot where Murtagh teaches Harry some of his unique magic techniques is really cool. It shows how different magical systems can interact. For example, in a fanfic I read, Murtagh showed Harry how to use a spell that was common in his world but unheard of in Hogwarts.
One interesting possibility is a universe where Voldemort won. It would be a dark and desolate world. The remaining Order of the Phoenix members would be in hiding, and Hogwarts would likely be a place of indoctrination rather than learning. Another could be a universe where Harry was sorted into Slytherin. He might have had a very different set of friends and challenges, and perhaps a more complex relationship with Draco Malfoy.
One possible plot could be Harry's discovery of his unique heritage. Maybe he starts having strange time - related powers, like being able to briefly stop time to escape danger or to look into the past to find clues about Voldemort's weaknesses. Another could be his interaction with other time - related magical beings. For example, he might meet an ancient time - keeper who guides him in understanding his role as the son of the time god.
One possible interesting plot could be Sakura time - traveling to a point where Kakashi is a young ninja still struggling with his past. She could use her modern - day knowledge and medical skills to help him in ways that change the course of his life. For example, she might be able to prevent some of the traumas he endured. Another plot could involve Sakura traveling to a future where Kakashi has become a Hokage, and she has to adapt to this new world while also dealing with her relationship with an older, more experienced Kakashi. A third plot could be Sakura getting stuck in a time loop that always brings her back to a crucial moment in Kakashi's life, and she has to figure out how to break the loop while also making sure Kakashi's future is secure.
There are fanfictions where Hermione travels to a time when Severus is a young student and she becomes his mentor of sorts. She teaches him things that he would have never learned otherwise, like advanced Muggle - friendly magic. This creates a unique bond between them that defies the normal timelines and relationships in the Harry Potter universe.
A really intriguing plot is when Hermione time - travels to save Bellatrix from becoming evil. It challenges the whole idea of good and evil in the wizarding world. Another is when Bellatrix travels forward in time and meets Hermione in a modern - day Hogwarts. It's interesting to see how Bellatrix reacts to the changes in the wizarding world.
In some fanfictions, like 'The Unforeseen Consequences of Time Travel', a plot twist could be that when Harry and Hermione go back in time to save someone, they end up causing more harm than good. This completely flips the expected outcome on its head and makes for a very engaging read.