In some fanfiction, the Avengers could go back in time to change a small event that had a huge butterfly effect on the future. For instance, they might go back to when Loki first arrived on Earth and try to make him an ally instead of an enemy. This would require them to convince Loki, who is very distrustful, and also deal with the potential consequences of altering his path in the timeline.
One possible plotline could be that Harry and Severus go back in time to prevent certain tragedies from happening in the wizarding world. For example, they might try to stop Voldemort's rise to power much earlier. Another could be that they go back to change their own relationship. Maybe in the original timeline they were enemies, but now they have a chance to become allies or even friends.
Well, a cool plotline might involve the main character introducing modern ideas to the Marauders' time. Maybe they bring new spells or ways of thinking that completely change the dynamic at Hogwarts. It could be really exciting to see how the Marauders and their friends react to these new concepts.
They might go back in time and accidentally change something that has a huge ripple effect on the future. So then they have to scramble to fix it before they return to their own time. They could also go back in time to learn some long - lost magic from an ancient wizard or witch and bring that knowledge back to their time to defeat a new dark force.
In this type of fanfiction, a great plotline could be Sansa influencing the political landscape much earlier. She could use her knowledge of how certain houses would act in the future to her advantage. For example, she could form early alliances with houses that she knows will be important later. She might also try to change the way her family views certain people. Maybe she could convince her father to see Littlefinger's true nature much sooner. This would not only protect her family but also change the entire power dynamic in Westeros.
One interesting plotline could be Castiel going back in time to prevent a major disaster in the past that has far - reaching consequences for the present. For example, he might stop a powerful demon from making a deal that would later lead to the death of many hunters.
In some 'Vegeta goes back in time' fanfictions, Vegeta might go back to the time when the Saiyans were still under the rule of King Vegeta. He could try to change the Saiyan culture from being so war - like and brutal. He might introduce new ways of thinking and training that would not only make the Saiyans stronger but also more united. This could lead to a completely different future for the Saiyans, where they don't get wiped out by Frieza and instead become a more dominant force in the universe. He could also form different alliances with other races much earlier, which would change the power dynamics in the galaxy.
One interesting plotline could be Snape going back in time and trying to prevent the death of Lily Potter. He might use his knowledge of the future to change certain events that led to her demise. Another could be him attempting to correct his own past mistakes, like his relationship with James Potter. Maybe he goes back and tries to befriend James earlier to avoid all the enmity that grew between them over the years.
Perhaps he goes back in time and decides to train himself better. He could focus on mastering his emotions from the start, which might change the entire course of the Star Wars saga. He might also try to save his mother in a more effective way, without succumbing to anger and hatred.
One interesting plotline could be Inuyasha going back in time and accidentally changing something small that has huge consequences for the future. For example, he might prevent a certain event that led to him meeting Kagome in the first place, and then has to find a new way to get back to his own time while also dealing with the changes he made.
One interesting plotline could be Natsu going back in time and accidentally changing something that has a huge impact on the future of Fairy Tail. For example, he might prevent a key member from joining the guild, and then he has to figure out how to fix it.