Maybe in the 'manga deku goes back in time fanfiction', Deku goes back to the time before he got his quirk. He then tries to understand the origin of quirks better, using his knowledge from the future. This could lead to him developing his quirk in a completely different way, perhaps even a more powerful way. He might also use this time - travel opportunity to befriend some characters who he didn't have a chance to in the original timeline.
In this fanfiction, Deku could go back in time to a crucial point in the history of heroes. For example, he might go back to the time when All Might was just starting out. He could then help All Might in ways that would change the future of hero society. He could share his future knowledge about the villains' plans, which would give All Might an edge in fighting them. This would not only change All Might's story but also the entire hero society that Deku knows in his original time.
In addition to the main characters, there could be some minor characters from the manga. For instance, the teachers at UA like Aizawa. Aizawa might notice that something is different about Deku and start to investigate. There could also be villains from the past that Deku has to face again, but this time with his future knowledge. Characters like Stain could be interesting to include as Deku might try to change the way he dealt with Stain in the original timeline.
Well, one interesting plot point could be Deku using his knowledge of future events to prevent certain tragedies. For example, he might stop a major villain attack before it even starts, saving many lives. Another could be his interactions with past versions of his friends and how he has to be careful not to change things too much.
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.