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 that the Hulk travels back in time to the moment when he first became the Hulk. He could try to prevent it from happening, which would change the entire course of his life and the Avengers. Maybe he teams up with a younger version of Bruce Banner to find a way to control the transformation without the need for the Hulk to exist in such a violent form.
Well, in some fanfictions, the time travel might go wrong in a way that it starts to merge different timelines. So the Avengers not only have to deal with Thanos but also with the chaos of colliding timelines. For example, characters from different points in time start interacting in strange ways, like an older version of Iron Man meeting his younger self. This can lead to a lot of interesting character development and conflict resolution.
Well, perhaps the kids travel to a time when the Avengers were just being formed. They meet the younger versions of their heroes and end up having to help them in a mission that was previously unrecorded. This would add a new layer to the Avengers' backstory and give the kids a chance to prove themselves. They could also learn valuable lessons from the early Avengers about teamwork and sacrifice.
A third plot could involve her time traveling to save a member of the Avengers who was crucial to the team but died in her future. She has to figure out how to blend in with the past timeline, deal with the Avengers not believing her at first, and then work together to stop the threat that originally killed that member.
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.