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.
Well, perhaps a plot twist is that Fem Harry Potter's magical abilities take on a different form at Beauxbatons. Instead of the typical Gryffindor - like courage - driven magic, she starts to develop a more refined and elegant form of magic influenced by the Beauxbatons environment. And then, she might get caught in the middle of a power struggle between two powerful Beauxbatons professors who each want to use her for their own purposes. Also, it would be cool if she uncovers a hidden prophecy about herself that was hidden in the Beauxbatons library all along.
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.
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 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.
One interesting plot twist could be that Thor goes back in time and accidentally changes something that makes him lose his powers. For example, he might prevent the event that led to him getting Mjolnir, and then has to find a new way to be a hero without his trusty hammer. Another could be that he goes back and meets a younger version of Loki who doesn't yet hate him, and he has to figure out how to keep it that way so that their relationship in the future is different.
Maybe he goes back in time and meets a younger version of his enemy. Instead of fighting right away, he tries to understand what made the enemy turn bad in the first place. This changes the whole future dynamic between them when he returns to his own time. He might find out that there was a misunderstanding that led to all the hatred.
One interesting plot twist could be that when Lucy goes back in time, she discovers that her family has a much darker secret than she ever knew. Maybe they were involved in some sort of magic that was considered forbidden in the past. Another could be that she accidentally changes the future in a way she didn't expect, like making her friends not become the people she knew them as. And perhaps she finds out that an enemy in the present was actually once an ally in the past, and she has to figure out what went wrong to turn them into an enemy.
One interesting plot twist could be that Naruto, with his knowledge of the future, tries to prevent certain tragedies but ends up causing new ones. For example, he might try to save a character too early and disrupt the natural flow of events, leading to unforeseen consequences.
One possible plot twist could be that when they go back in time, they accidentally change something that has a huge impact on the present. For example, they might prevent the sorting hat from putting them in their original houses. This could lead to a completely different dynamic at Hogwarts, with different friendships and rivalries forming.
Well, in some 'Harry Potter goes back in time' fanfics, Harry might go back to his first year at Hogwarts. He could then use his more advanced magical skills (from his future experiences) to solve the mystery of the Philosopher's Stone in a completely different way. This might involve him befriending different people earlier or even changing the course of how the challenges in the dungeon were set up.