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 Yoda traveling back in time to train Anakin Skywalker differently, thus preventing his fall to the dark side. Maybe he goes back to the very beginning of the Jedi Order to gain more knowledge about the Force and bring it back to his own time.
One idea could be Shinji traveling back in time to prevent the Second Impact. He might try to change the actions of his father that led to it. Another could be Asuka traveling to a future where the Evangelions have been completely forgotten and she has to figure out how to get back to her own time.
Another plot could be a character from the future coming back to the present in Shameless. They could be trying to stop a future disaster that is somehow linked to the Gallagher family. This would involve a lot of mystery and intrigue as they try to figure out what the future event is and how to prevent it while dealing with the chaos of the present - day Gallagher family.
One idea could be Goten traveling back to the Saiyan - Saiyan war era. He could end up changing some small events that have big impacts on the future, like uniting some Saiyan clans earlier. This would create a whole new timeline where relationships between characters are different.
Maybe a character travels forward in time and sees a dystopian version of the ACOTAR world. They then have to go back and try to prevent the events that led to that future. They could face many challenges along the way, like not being believed or having to work against powerful forces who benefit from the future they saw. It would be a race against time to save their world.
A possible plot for 'Haikyuu time travel fanfiction' is that Tsukishima travels back to his childhood when he first started to dislike volleyball. He could then understand why he developed that negative feeling and change his attitude from the start. This would lead to a much stronger Tsukishima in the present day. Another idea is that a team manager travels back in time to recruit a different set of players for the team, creating a completely different team dynamic. Maybe they could even bring back an old - school player who had unique skills that could revolutionize the team's playing style.
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.