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.
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 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.
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, 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.
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 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.
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.
They could also travel to a time during one of the great ninja wars. They might get involved in battles they previously only read about in history books. Sasuke could team up with his ancestors and Sakura could learn new medical techniques from the medics of that time. This would not only change the course of the war they are in but also bring back new knowledge to their own time.
One plot idea could be that Naruto and Sasuke keep reliving the day of the final battle. Each time loop, they gradually discover hidden secrets about their pasts that change their relationship. For example, they might find out that there were other forces at play that made them enemies in the first place.
One idea could be Sasuke traveling back to the Uchiha massacre. He might try to prevent it, which would completely change the Naruto storyline. This could lead to him having to face off against Itachi in a different context, not as an avenger but as a protector of his family.