It could still be Orochimaru. Even if Sasuke travels back in time, Orochimaru's thirst for power and knowledge wouldn't change. He would still be plotting to gain power, and Sasuke, with his knowledge of Orochimaru's future actions, would be in a position to stop him. Orochimaru's experiments and his desire to take over the bodies of others make him a formidable antagonist.
Perhaps Sasuke travels back to the very beginning of the series. He could choose to befriend Naruto right from the start, which would completely alter their dynamic. He could also use his advanced skills to quickly rise in the ninja ranks and start to influence the political situation in the Hidden Leaf. This would lead to a very different story as he tries to prevent future tragedies and build a stronger, more united ninja world.
She could meet the founders of Hogwarts. Imagine Hermione having a conversation with Rowena Ravenclaw about the importance of knowledge and wisdom. She might also meet Godric Gryffindor and learn about the true meaning of courage from him. Another person she could meet is Salazar Slytherin. It would be very interesting to see how she would interact with him, given his views on blood purity.
The concept of Sakura going back in time in relation to Sasuke in a fanfiction offers a great opportunity for character growth. Sakura could go back and train with Sasuke from an earlier age. This would not only strengthen their bond but also make Sasuke more open to Sakura's presence. It could also mean that Sakura becomes a more powerful ninja in this new timeline, being able to stand beside Sasuke as an equal rather than always chasing after him.
Another theme is self - discovery. By going back in time and seeing their past selves, they learn more about who they are and how they've changed. Naruto might realize how much he's grown in terms of his ninja skills and his relationships. They also understand their roles in the grand scheme of things better.
Perhaps Sasuke goes back to the very beginning of his ninja training. He decides to learn different jutsu that he didn't focus on before. He could also try to understand the political situation of the village better. Since he has seen the consequences of certain actions in the future, he aims to change things for the better. For example, he might try to improve relations between the Uchiha and the rest of the village much earlier. This would involve interacting with the elders and trying to find a compromise. It could also lead to a different path for Itachi as well.
He might meet the future versions of Naruto and Sakura. Their relationship with him could be very different in the future, perhaps they are now leaders of the village and Sasuke has to adjust to their new status.
In some cases, the antagonist could be Esme herself, or rather a version of her from the past. The past Esme might be more naive or self - centered and not understand the implications of Esme's actions in the past. The present Esme is trying to make positive changes, but the past version of herself keeps getting in the way, either through her own actions or by influencing the people around her in a negative way.
One possible plot could be that Esme travels back to a time when she was about to make a crucial life decision. For example, she might go back to the moment before she met her significant other. In the past, she could then have the chance to see things from a different perspective. Maybe she realizes that the decision she made back then was actually the best one, or she might find out new things about herself or the people around her that she didn't know before, which could lead to a whole new set of adventures and self - discovery in the past and ultimately change her view of her present life.
One common theme is redemption. Percy often goes back in time to correct past mistakes, either his own or those of others, seeking a kind of redemption. For example, in some stories, he tries to save characters who died in the original timeline. Another theme is self - discovery. When he's in a different time, Percy learns more about himself and his powers. He might discover new abilities or a different side of his personality.