Well, a possible plot twist is that when Harry travels to his parents' time, he realizes that there is a much bigger conspiracy at play. The Dark forces might be planning something way more sinister than he thought, and his presence in the past might be the key to either stop it or unknowingly help it. And what if his time - travel causes some sort of time - loop? Every action he takes in the past just keeps repeating in a cycle, and he has to find a way to break it. Or perhaps he meets his parents, but they don't believe he's their son from the future and think he's some sort of imposter or a mad wizard.
A really cool plot twist could be that Harry's time - travel actually sends him to an alternate version of his parents' time. In this version, the wizarding world has different rules or powers. For instance, the magic might be more elemental and less wand - based. His parents could also be part of a different social or political structure within the wizarding world. Maybe they are part of a secret group that is fighting against a different kind of dark magic that doesn't exist in Harry's own time. This would completely change Harry's understanding of the wizarding world and his place in it, and he would have to adapt quickly to survive and figure out how to get back to his own time.
One interesting plot twist could be that Minato travels back to a time when the Hidden Leaf Village is on the verge of a civil war. He uses his future knowledge to prevent it, changing the entire future of the village. Another could be that he travels to a time where his parents are still alive and has to deal with the complex emotions of seeing them again.
Well, a really interesting plot twist in an Eren travels back in time fanfiction could be related to relationships. Say Eren goes back and realizes that his actions in the past have a huge impact on his relationships with his friends. Maybe he accidentally causes Mikasa to not be as attached to him as she was in the original timeline. Or he could find out that Armin was supposed to be the one with the time - traveling ability all along, and his interference messes up Armin's future plans. Also, what if he goes back and discovers that the real enemy wasn't the Titans but some other force that was using the Titans as a distraction, and his time - traveling journey is the only way to expose this truth.
A plot twist could be that Sakura travels back in time and realizes that the world she thought she knew was all a lie. The history books in her time were wrong. She could find that the origin of her magical powers was something very different from what she was taught. Also, she might go back to a time when there was a powerful artifact that was lost in her own time. However, when she tries to retrieve it, she discovers that it has a dark side that she didn't expect. This could lead to a moral dilemma for her, whether to keep it or leave it be.
One interesting plot twist could be Percy meeting his future self. His future self might have some dark secrets or a different view on the world, which could lead to a lot of internal conflict for Percy. Another twist could be Percy accidentally changing a major event in the past that then has a huge impact on the present of the demigod world. For example, if he stops a certain prophecy from being made, it could completely rewrite the power dynamics among the gods and demigods.
Perhaps when Bilbo travels back in time, he finds that the Ring has an unexpected origin or connection to other powerful artifacts. This could lead to a whole new set of challenges and adventures as he has to deal with these new revelations while still trying to destroy the Ring. For example, he might find that the Ring is part of a set of items that need to be dealt with together in a specific order.
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.
His presence could strengthen their relationship. If Harry were to save them from a dangerous situation, it might make them bond more over the shared experience. Or he could give them some advice that helps them understand each other better.
One interesting plot twist could be that when Harry travels back in time, he accidentally alters the events that led to Andromeda's relationship with the Black family. This could create a whole new set of family dynamics for her. For example, if Harry stops her from making a certain choice, it might lead to her being more involved with the dark side of the wizarding world initially, but then his presence in that time could also be the catalyst for her to change again and become a key figure in fighting against the dark forces in a different way than in the original timeline.
Perhaps Harry Potter travels back in time and ends up befriending a younger Bellatrix. He could try to influence her to not become a Death Eater. This would create a really complex situation because he has to balance not changing too much of the future while also trying to save her from the dark path. It could lead to some really intense moments where Harry has to hide his true identity and motives from her.
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.