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.
Well, perhaps Thor travels back in time and finds out that there was a prophecy about him that was hidden from him in the future. This could lead to a whole new set of adventures as he tries to figure out what it means and how it affects his future self. Or, he could go back and meet an ancestor who was a great hero, but also learn that this ancestor had a dark secret that could have implications for Thor's own heroism.
A possible plot twist is when Thor goes back in time and discovers that his father Odin was actually from a different realm originally. This revelation could change Thor's entire understanding of his family history and his place in the cosmos. Also, he could go back to a time when Asgard was at war with a long - forgotten enemy, and he has to use his knowledge from the future to either prevent the war or win it in a different way than before.
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.
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 possible plot twist could be that Katniss goes back in time and prevents the Hunger Games from ever starting. Maybe she meets the people who first came up with the idea and manages to change their minds. Another could be that she goes back to a time before her father died and is able to save him, which would completely change her family's situation and her own personality development. A third plot twist might be that she goes back to a crucial moment in the Games and alters the outcome in a way that surprises everyone.
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 possible plot twist could be that when they go back in time, they accidentally change something that has a huge impact on their future relationships. For example, Naruto might unknowingly prevent Sasuke from leaving the village in the past, which could completely change how they interact in the present they return to. Another could be that they find out a hidden truth about their parents or the village's past that was covered up, like a secret alliance that their predecessors were involved in.
A really interesting plot twist could be that Steven goes back in time and discovers that the Diamonds had different motives than he originally thought. Maybe they were being influenced by some other force in the past, and by going back, he uncovers this mystery. Also, he could find out that some of the things he thought were his friends' ideas were actually planted by an outside source long ago. This would completely change his understanding of the relationships and history in the Steven Universe world.
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, sometimes in these fanfictions, a plot twist might be that Sirius goes back in time and discovers a hidden truth about his family. Maybe he finds out that there was a secret alliance or a hidden enemy within the Black family that was influencing events all along. This could completely change his perception of his family's past and his place in it. Also, he could go back in time and end up being involved in a different kind of magic that was lost in his original time, like an ancient form of magic that has the power to either save or destroy the wizarding world.
One possible plot twist could be that Jon Snow goes back in time and accidentally changes something that leads to a completely different future for the Stark family. For example, he might prevent Ned Stark from going to King's Landing, which would change the power dynamics in Westeros entirely.