Perhaps Sasuke travels back to the very beginning of the series. He could choose to befriend Naruto right from the start, which would completely alter their dynamic. He could also use his advanced skills to quickly rise in the ninja ranks and start to influence the political situation in the Hidden Leaf. This would lead to a very different story as he tries to prevent future tragedies and build a stronger, more united ninja world.
She could go back in time to the Triwizard Tournament and use her knowledge of future events to prevent Cedric's death. Maybe she figures out a way to expose Barty Crouch Jr. much earlier. Another plot idea is that Hermione travels back to the time when the Marauders were at Hogwarts. She could interact with them, and perhaps try to influence Sirius Black's fate, making sure he doesn't end up in Azkaban. She might also try to learn more about Remus Lupin's lycanthropy from a different perspective.
Well, he could go back to the time when his crew first formed. Luffy might use his knowledge of the future to make their early adventures easier, like knowing exactly where the treasure is on certain islands. Or he could travel back to the time of the great pirate wars and try to stop some of the bloodshed. This would change the power dynamics in the world and create a whole new set of challenges and allies for him.
One idea could be that Bella travels back to a time when vampires first emerged. She could try to change the history of vampire - human relations, maybe prevent some of the early vampire - made disasters. Another idea is she goes back to a time in her own family's history and discovers secrets that shape her present. For example, she might find out that her family has some hidden connection to the supernatural world long before she became involved with Edward.
One idea could be Naruto going back in time and preventing the Uchiha massacre. He could use his knowledge of future events to stop Itachi from having to make that tragic choice.
Well, perhaps Goku travels back in time to meet his long - dead parents. He could have conversations with them, learn about his family's history in a more personal way, and gain new insights that help him in his present - day battles. Or he might go back to the time when the Saiyans were still on their home planet, and try to change the fate of the Saiyan race by warning them about their impending doom.
One idea could be that Natsu goes back in time and accidentally changes something small that has a huge impact on the future. For example, he might prevent a key encounter that led to the formation of his guild, Fairy Tail, and then has to find a way to fix it.
One idea could be that Sasuke goes back in time and tries to prevent the Uchiha massacre. He could use his knowledge of the future to stop Itachi from carrying out the order. Another could be that he goes back to his early days at the ninja academy and tries to change his relationship with Naruto from the start. Maybe he would be more open and friendly, which could change the whole dynamic of their rivalry.
Perhaps Lucy time travels to a prehistoric era. She has to quickly adapt to survive among the dinosaurs. Along the way, she discovers that there are ancient secrets or powers hidden in that time that have an impact on her own time. She then has to figure out how to use this knowledge when she returns home.
It could still be Orochimaru. Even if Sasuke travels back in time, Orochimaru's thirst for power and knowledge wouldn't change. He would still be plotting to gain power, and Sasuke, with his knowledge of Orochimaru's future actions, would be in a position to stop him. Orochimaru's experiments and his desire to take over the bodies of others make him a formidable antagonist.
A plot could involve Naruto and Sasuke being stuck in a time loop where they are the only ones aware of it. Each loop, they find that small changes in their actions lead to big differences in the outcome. For example, in one loop, if Naruto decides to train differently, it changes how Sasuke views their rivalry. They could be trying to break the loop by fulfilling a certain prophecy or task that they gradually uncover.