One idea could be Big Boss traveling back to the time before the events of Metal Gear Solid 3. He could use his future knowledge to change certain things in the past, like preventing the death of a certain comrade. This would create a whole new timeline and new challenges for him.
Another plot could involve Big Boss traveling to a future he didn't expect. Maybe a future where his actions had far worse consequences than he ever imagined. He then has to find a way to set things right, which might involve teaming up with unexpected allies from that future time.
Maybe a crossover with a post - apocalyptic world. Big Boss enters a world like that of The Walking Dead. He has to use his survival and combat skills in a completely different scenario. He could form an alliance with some of the survivors there, teaching them his ways of combat and in return, learning about their unique ways of surviving in a world full of zombies. It would be a great mix of two different universes.
One idea could be Hiro traveling back in time to prevent the accident that led to Tadashi's death. He could use his knowledge of the future to stop the fire from happening, thus changing the entire course of the story. This would also impact the formation of Big Hero Six as they might not have come together in the same way without that tragic event.
A great plot for this fanfiction could be that Big Boss time - travels to different eras within the Metal Gear Solid universe. In one instance, he travels to a time when the Philosophers were just starting to form and has to deal with the political intrigue at that level. He could try to influence their decisions to shape a different future for the world. Another aspect could be that while time - traveling, he meets alternate versions of himself. These encounters could lead to self - discovery and internal conflicts as he realizes the different paths he could have taken. It would also be cool if he had to team up with some of his former enemies in a different time period to fight a common threat that only exists because of time - travel anomalies.
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.