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.
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.
The plot could involve Sakura getting a chance to relive parts of their past. She might try to save Sasuke's family, which could change Sasuke's entire motivation and path. This would have a huge impact on the Naruto universe as we know it.
It could be a time - traveling version of Orochimaru. He might have his own plans for the past and Sasuke's interference could put them at risk. So, he tries to stop Sasuke from changing the past in a way that would undermine his future goals.
One interesting 'Sasuke back in time' fanfiction could be where he goes back to the Uchiha massacre. He might use his knowledge from the future to prevent it. Maybe he tries to change Itachi's mind or find a different solution to the conflict within the Uchiha clan.
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.
One idea could be that Sasuke goes back in time and tries to prevent the Uchiha massacre. He could use his knowledge of the future to stop Itachi from carrying out the order. Another could be that he goes back to his early days at the ninja academy and tries to change his relationship with Naruto from the start. Maybe he would be more open and friendly, which could change the whole dynamic of their rivalry.
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.
Another reason might be to stop a greater threat. Maybe there was a hidden enemy in the past that they didn't know about before, and by traveling back, they can stop this enemy from becoming too powerful. In some fanfictions, they discover that a small event in the past was the root cause of a much larger and more dangerous future situation.
There's also 'Back to the Beginning'. In this fanfiction, Naruto and Sasuke go back to their childhood days. It's really interesting to see how they interact with their younger selves and try to change things for the better. They face challenges as they have to be careful not to disrupt the timeline too much.