Perhaps in the fanfiction, when time is turned back, Hermione decides to focus on different areas of magic. Instead of being all about books and rules, she starts to explore dark magic in order to stop Voldemort before he rises. This could put her at odds with Harry and Ron. Also, if time is turned back far enough, Dumbledore could be a student again. He might make different choices that change the whole wizarding world's future, like not getting involved with Grindelwald, which would have a huge impact on the events leading to Voldemort's rise.
One interesting plot twist could be that Harry goes back in time and accidentally changes the sorting of some of the characters. For example, Draco Malfoy could end up in Gryffindor instead of Slytherin, which would completely change their relationship and how events unfold in the wizarding world.
In this type of fanfiction, an interesting plot twist might be that Harry, as a girl, is underestimated by many of the male characters at first. But then she uses this to her advantage. She could pretend to be meek and mild while secretly gathering information and planning strategies. Also, her transformation could lead to a change in the way Hogwarts is run. Maybe she starts a movement for more equality between male and female students, which causes a stir among the old - fashioned teachers.
A cool plot twist might be that Fem Harry goes back in time and discovers that her parents were involved in a secret mission that was never revealed before. This could lead to her having to deal with the consequences of uncovering that truth. Also, she could find out that Voldemort had a different origin or a hidden weakness that was lost in history. As she tries to use this knowledge to defeat him earlier, she might face opposition from some of the older wizards who don't believe her or are afraid of the changes she could bring. It could also be that she changes the way magic is taught at Hogwarts, introducing new spells or techniques from the future that create a whole new set of challenges and opportunities for the students.
One interesting plot twist could be a character who was thought to be a minor sidekick turns out to be a key figure in a secret wizarding society. This unexpected revelation changes the whole dynamic of the story.
One interesting plot twist could be when a character who was originally a minor antagonist in the original series becomes a key ally in the 'back to the past' scenario. For example, in some fanfictions, Draco Malfoy might end up joining Harry's side early on, changing the whole dynamic of the story. Another could be when a major event from the original series is prevented, like Sirius Black's death. This creates a whole new set of consequences and relationships that the author can explore. And sometimes, the way magic is used in these fanfictions can be a plot twist. Maybe a character discovers a new type of magic that wasn't in the original books, which gives them an unexpected advantage in the 'back to the past' world.
One interesting plot twist could be that every time Harry repeats the day, a different character becomes aware of the time loop. For example, in one loop it's Hermione who figures it out and tries to help Harry break the loop in a very scientific way, using her vast knowledge. Another could be that Voldemort somehow gets a hint of the time loop and tries to use it to his advantage, like setting up different traps for Harry each time the loop restarts.
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.
A possible plot twist is that when Harry goes back, he realizes that the Marauders are not exactly as he imagined them. There could be secrets within the group that he uncovers. For instance, James Potter might have had a different side to him that was not shown in Harry's original understanding. This discovery could make Harry question his own identity and his relationship with his father. Also, the time - travel could be a trap set by a dark force. Harry might find himself in a situation where he has to fight against this dark plot while trying to maintain the integrity of the timeline.
One interesting plot twist could be the seer seeing a future where Voldemort is actually a puppet of a more powerful dark wizard hidden in the shadows. This would completely change the power dynamics in the wizarding world.
One interesting plot twist could be that Harry discovers he has the ability to communicate with the Basilisk on a deeper level than just Parseltongue. Maybe he can see through the Basilisk's eyes and use this to his advantage in uncovering secrets within Hogwarts that even Dumbledore doesn't know. Another could be that the Basilisk is not truly a villain but was being controlled by a dark force all along, and Harry has to find a way to break that control and free the Basilisk.