Well, perhaps in the fanfiction, Master Chief goes back to his own past. He could meet his younger self and try to guide him. He might teach his younger self some combat techniques he learned later in his life, or give him some advice about dealing with the UNSC politics. This would create an interesting dynamic between the two versions of himself.
One possible plot could be Master Chief going back to a time before the Covenant war. He could use his knowledge and skills to prevent certain events that led to the war. For example, he might stop the discovery of the Halo rings by the Covenant in the first place, which would change the entire future of the galaxy.
In this fanfiction, Master Chief could go back to a time when humanity was just starting to explore the stars. He could be the one to introduce advanced technology that would make humanity more powerful. He might also change the way humanity approaches alien species, leading to better relations and preventing future conflicts. This would rewrite the whole history of humanity in the Halo universe.
He could meet Dr. Halsey when she was just starting her research on the Spartan program. Master Chief could give her some insights from the future, which might change the way the program is developed.
One possible plot could be that Kagome goes back in time and tries to change a major event in the past. For example, she might try to prevent a big battle that had a lot of casualties. She could use her modern knowledge and skills to her advantage in the past, like using first - aid knowledge to save lives.
Well, one possibility could be that Kurama goes back to a time when he was first captured by the shinobi. He might try to change things so that he doesn't end up in their hands initially. He could use his knowledge of the future to outwit his captors and escape much earlier, and perhaps even start a new life in a different part of the world.
Well, in the fanfiction, Itachi could go back in time and decide to train his younger self more rigorously. This would not only make him stronger but also change the way he approaches his future tasks. He might also try to influence Sasuke at an earlier age, guiding him in a different direction than in the original timeline. And he could befriend some characters earlier to build a stronger support network for the Uchiha clan.
Maybe Yusei goes back in time to prevent a great disaster that happened in the past. He could use his knowledge from the future to change certain events and save lives.
In a 'kaneki goes back in time fanfiction', perhaps Kaneki goes back to when he was still a normal college student. He then decides to study more about ghouls secretly. He could use this knowledge to try and find a way to co - exist peacefully between ghouls and humans from the very start. He might start by trying to change the views of some key people in the CCG, using his understanding of their future actions.
Well, one possible plot could be that Zuko goes back in time and tries to prevent the Fire Nation's aggression from the start. He could use his knowledge of the future to change things for the better, like trying to influence his father in a different way to avoid the war.
One possible plot could be Serena going back in time to prevent a major disaster in her world. For example, she might go back to stop an evil force from being released. In the past, she could meet younger versions of her friends and family, and they might not recognize her at first. She would have to use her knowledge from the future to try and change things for the better.
Perhaps Elsa goes back in time to meet her parents again. She could learn more about her powers from them, as they might have known more than she initially thought. This encounter could also give her a chance to say goodbye properly, which she didn't get the first time. And it could lead to her having a better understanding of her place in the kingdom and her family.
In a 'qrow goes back in time' fanfiction, one possible plot could be that Qrow goes back to a time before a major tragedy in his life. He could then use his knowledge of the future to prevent it from happening. For example, if there was a battle that led to the loss of many of his friends, he might train harder or gather more allies in the past to change the outcome.