Well, a plot could involve Ben going back to the time when the Omnitrix was first created. He could witness the creation process and discover some hidden secrets about the device that could help him in his present - day battles. Maybe he has to team up with the original creators in the past to fix a glitch in the Omnitrix that is causing problems in the future. Another plot might be that Ben travels to a post - apocalyptic future time where the world has been destroyed by an unknown force related to his alien forms. He then has to travel back in time to prevent this from happening.
One idea could be Ben traveling back in time to meet his younger self and having to hide his identity while trying to prevent a future villain from getting a powerful weapon in the past. Another could be Ben traveling to different time periods in the Ben 10 universe to gather artifacts that are needed to stop an all - powerful time - traveling enemy in the present. Or he could accidentally end up in a time where the Omnitrix doesn't exist yet and has to find a way back while dealing with the different power dynamics of that era.
One idea could be Ben going back in time to prevent an alien invasion that in the original timeline led to the destruction of his hometown. He could use his ultimate alien forms in creative ways to outwit the invaders before they even start their attack.
One idea for a Ben 10 Ben and Young Gwen time travel fanfiction could be that they travel back to the time when the Omnitrix was first created. They could witness the origin of the device and perhaps even meet the aliens who had a hand in its creation. This could lead to them learning secrets about the Omnitrix that they didn't know before, which could help them in their present - day battles.
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.