One possible plot twist could be that Optimus discovers he is the cause of an event in the past that he was trying to prevent. For example, his presence might accidentally lead to the earlier awakening of a powerful enemy. Another could be that he meets a younger version of himself and has to convince his past self to trust him without revealing too much about the future.
Well, perhaps a plot twist could involve Optimus being sent back to a time when the Autobots and Decepticons had a different relationship. He might find out that they were once allies or at least on the verge of peace, and his arrival disrupts that fragile balance. Or he could be sent back to a time when there was a hidden Autobot or Decepticon technology that was lost in his original timeline, and he has to figure out how to use it without changing history too much.
A really interesting plot twist could be that Optimus is sent back in time but his memories are slowly fading. He has to race against time not only to complete his mission but also to remember why he was sent back in the first place. As he tries to piece together his memories, he might accidentally make changes to the past that he didn't intend to. Also, he could discover that a key Autobot or Decepticon from his own time has a completely different role or personality in the past, which throws him off and makes his task even more difficult.
His arrival could have a huge impact. He might introduce new technology or fighting techniques to the Autobots in the past. This could change the outcome of battles they were previously losing. For example, if he shows them a more effective way to defend against a Decepticon attack.
One possible plot twist could be that Ragna discovers he is the cause of a major historical event that he previously thought was just a part of history. For example, he might find out that his actions in the past led to the rise or fall of a powerful kingdom.
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.
One possible plot could be that Ron goes back to his Hogwarts days. He could use his knowledge of the future to prevent certain bad things from happening, like maybe he stops a particular dark magic attack on the school. He might also try to improve his relationships, for example, he could be nicer to Hermione from the start and this could change the whole dynamic of their friendship and more.
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.
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 possible plot twist could be that Levi travels back to a time when the Survey Corps was first formed. He might face the initial challenges all over again, but with his future knowledge. For example, he could know which recruits would be most valuable in the future and focus on training them earlier. Another twist could be that he travels back to a time before the fall of Wall Maria. He might try to prevent certain events that led to the fall, like uncovering a hidden conspiracy within the military that was planning to let the Titans in. It could also be that he goes back to a time when his past self was making a crucial decision, and his future self has to influence that decision without changing too much of the timeline, as that could lead to unforeseen and potentially disastrous consequences.
The plot might involve Percy going back in time and getting involved in a love story that was lost to history. He could fall in love with someone from the past and then have to make a difficult choice when it comes time to return to his own time. It could also be that he has to use his modern knowledge to solve problems in the past that no one else could figure out, which then changes the course of history in small but significant ways.
Well, perhaps a major plot twist is when Percy goes back in time and realizes that the events he thought were accidents in his own time were actually carefully orchestrated by a character from the past. This would add a whole new layer of mystery and conspiracy. Also, he might find that a prophecy he was trying to prevent in his time was actually meant to happen in the past for a greater good, which would be a huge turn for his mission.