A possible plot in Tsunade time travel fanfiction could be her journeying back to the time when Orochimaru was still a loyal Leaf ninja. Tsunade, knowing what he would become in the future, could try to influence him in a positive way. She might offer him more support and understanding, perhaps even guiding his research into more ethical directions. This could change the entire course of Orochimaru's life and the subsequent events in the Naruto world.
One idea could be Tsunade traveling back to the time when the Hidden Leaf was first founded. She could use her medical and combat skills to help the early settlers deal with diseases and threats from other villages. Another plot could be her going back to the time of the Third Great Ninja War and changing the fates of some of the fallen ninjas with her knowledge of future events.
Maybe they travel back to a time when Tsunade was just starting to develop her medical ninjutsu. Naruto, with his understanding of modern medical knowledge (from Sakura perhaps), could help her make great leaps in her medical skills. They could also team up to deal with a powerful enemy that was a threat at that time. Naruto's strength combined with Tsunade's budding abilities would make for an exciting story.
Well, she could go back in time and end up training her younger self. Tsunade could teach her younger version all the things she learned over the years, especially about handling emotional pain and making tough decisions. Also, she might go back to a time when Orochimaru was still a good friend. Tsunade could try to prevent him from going down the dark path. She could use her knowledge of his future actions to stop him from leaving Konoha and getting involved in all those evil experiments.
One idea could be a modern-day character traveling back in time to the Hakuoki era and getting involved in the political intrigue. They could use their modern knowledge to try and change the course of events. Another plot could involve a character from the Hakuoki world traveling to the future and being shocked by the technological advancements. Maybe they struggle to adapt to the new world but also find new ways to bring back knowledge to their own time.
Perhaps Rex travels to a time where dinosaurs and humans co - exist. He has to figure out how this is possible and if it's a threat to the future. Or he could go back to the time of the Renaissance and become an apprentice to a famous artist, using his modern perspective to influence the art world in ways that have a ripple effect through time.
One idea could be that Tony Stark travels back in time to prevent the creation of certain dangerous technologies that later cause chaos in his timeline. He could use his genius to subtly alter the course of scientific development.
One idea could be that the Avengers go back in time to stop Thanos from ever getting the Infinity Stones in the first place. They might have to deal with different versions of themselves and the challenges of changing the past without ruining the future.
One idea could be Sasori traveling back to his childhood and trying to change his path towards becoming a puppet. He might attempt to save his parents and avoid the tragedies that led him to his cold-hearted ways. Another plot could be Sasori traveling to a future where the Akatsuki has achieved its goals, only to find that the world is in a state he didn't expect, so he has to go back and change the Akatsuki's plans. Or he could travel to different ninja villages' pasts to steal their most powerful puppet - making secrets.
In some narusasu time travel fanfiction, Naruto might go back to the time when Orochimaru was first targeting Sasuke. He could then use his skills and knowledge to keep Sasuke safe and away from Orochimaru's influence. A different plot could be Sasuke traveling to a time when Naruto was ostracized as a child and being the one to support Naruto, which would change their relationship dynamic from the start. Also, they could travel to a time during a major battle and use their combined future knowledge to turn the tides in their favor.
One idea could be Madara traveling back to the era of the Sage of Six Paths. He could try to change the very foundation of ninjutsu, which would create a whole new timeline. Maybe he would attempt to gain the power of the Sage for himself much earlier in history.
One plot idea could be Naruino traveling back in time to prevent a great disaster in their world. Maybe a powerful enemy once defeated them in the past, and by going back, they can change the outcome. Another could be traveling to the future to see how their actions in the present have affected the world. For example, they might find that their current choices led to a utopia or a dystopia.