A really cool plotline might be Naruto training his younger self. Since he has all his future skills and knowledge, he could teach his younger self advanced jutsu much sooner. This would make his younger self a much more powerful ninja at an earlier age, and could potentially change the power dynamics in the ninja world.
Another plotline could be Naruto influencing the relationships between characters. He could try to bring Sasuke back to the village much earlier in the timeline, preventing a lot of the pain and conflict that occurred between them. Or he could help his parents, Minato and Kushina, in ways that would change the future of the entire Hidden Leaf village.
One interesting plotline could be Naruto using his future knowledge to prevent certain tragedies. For example, he might stop the death of his parents. He could also use his knowledge of jutsu to quickly become a much stronger ninja at a younger age and change the power dynamics in the village.
One interesting plotline could be Naruto using his future knowledge to prevent certain tragedies in the past. For example, he might stop the Uchiha massacre. Another could be him training his younger self more effectively, leading to a much stronger Naruto earlier on.
One interesting plotline could be Harry going back in time and preventing certain tragedies from happening in the wizarding world. For example, he might stop the death of his parents. Another could be him using his future knowledge to change the way Hogwarts is run, like improving the curriculum for Defense Against the Dark Arts.
One possible plotline could be Naruto using his knowledge from the future to prevent certain tragedies. For example, he might stop the death of a beloved character like Jiraiya.
Well, one possible plotline could be Naruto having to deal with newfound powers that come with this strange state while trying to keep it a secret from his friends. Another could be how his relationships, like with Sakura or Sasuke, are affected as they notice his different behavior.
One interesting plotline could be Naruto's struggle to adapt to the strict and secretive ways of the ANBU. He's so used to his more open and brash style, so it's a big change. For example, he might have trouble with the stealth aspect at first.
One possible plotline could be Naruto having to adapt his fighting style. Since he can't hear, he might rely more on his visual senses and his chakra sensing ability. For example, he could learn to read his opponents' lips during battles to anticipate their moves.
A third plotline could involve Hinata going back in time and having to deal with the consequences of changing the timeline. Her actions might create new problems or enemies. For instance, if she changes a major event, it could disrupt the balance of power among the ninja villages, leading to unforeseen conflicts that she then has to resolve.
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.
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.