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.
His sense of purpose might also change. In his original time, he might have been fighting for a certain cause. But in the past, he could discover new things to fight for or new values to uphold. Maybe he realizes the importance of preserving certain aspects of the past that were lost in his future, and this changes his overall attitude and personality as he becomes more of a preserver than a fighter in some ways.
Time travel forces Ragna to face his past, which can make him more self - aware. He may see the mistakes he made in the past and strive to correct them, leading to personal growth.
Revan sent back in time might completely change the course of galactic history. He could influence key characters in the past. Maybe he'd train a young Jedi who would become a great hero earlier than in the original timeline. Or he could interfere with the Sith teachings, leading to a different breed of Sith or even weakening the Sith order as a whole.
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.
Percy Jackson would be an obvious choice. His skills as a demigod and his experience with different quests would make him a great character to be sent back in time. He could use his water powers in unique ways in different historical settings.
In such a fanfiction, Percy might use his knowledge of the future to change some key events in the past. For example, he could prevent some of the earlier disasters or conflicts in the Percy Jackson universe. Maybe he'd try to stop the first encounter with a certain monster in a different way to avoid the chaos it caused.
In such a fanfiction, Darth Vader could change the course of history. He might prevent the rise of the Empire or alter his own fall to the dark side. For example, he could warn Anakin Skywalker about Palpatine's true intentions, thus changing the entire Star Wars timeline.
It could be an ancient and powerful Titan who was thought to be defeated long ago. Since Percy is in the past, this Titan could be in his prime and see Percy as a threat to his plans of ruling over the world again. For example, Kronos might be even more powerful in this past era and Percy has to find a way to stop him without the help of his modern - day friends.
Another theme is self - discovery. By going back in time, Tsunayoshi gets to see himself from a different perspective. He realizes his own growth and potential, and this helps him to become a stronger person both in the past and for his future self. In some fics, he discovers new abilities or talents that he didn't know he had before.