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 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.
Sasuke is also very popular. As Naruto's friend and rival, many fanfictions explore how their relationship would be different if Naruto had a second chance. For example, Sasuke might be more open to Naruto's friendship earlier due to Naruto's new approach influenced by his future knowledge.
Well, he could potentially change a lot of things. Maybe he would prevent some of the tragedies that happened in the original timeline. For example, he might stop the Uchiha massacre. He could also use his knowledge of future events and the power of the Juubi to make the ninja world a more peaceful place from the start.
The plot could also involve the other characters' reactions to this evil version of Naruto from the future. Some might be confused, while others might immediately sense the danger. Naruto could also face challenges in changing the past as there could be unforeseen consequences or time - related ninja techniques that prevent him from fully achieving his evil goals.
One possibility is that Naruto, with his godlike abilities, could completely rewrite the history of the ninja world. He could go back to the time of the Sage of Six Paths and learn from him directly. This would give Naruto an even greater understanding of chakra and ninjutsu. He could then introduce new forms of training and cooperation among the ninja villages, perhaps even preventing future wars. He could also ensure that the jinchuriki are treated better in the past, changing their fates for the better.
Well, in such a fanfiction, the characters might try to change past events. For example, Naruto could prevent some of the early tragedies in his village. Maybe he'd stop the Nine - Tails attack by somehow convincing his father not to sacrifice himself. This would change the whole future of the village and the relationships between characters.
One common theme is redemption. Naruto often uses his second chance from going back in time to redeem past mistakes, especially those related to his relationship with Sakura. For example, he might correct things he said or did that hurt her in the original timeline.
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.
Well, he would have a second chance at life. Since he was a baby, he could grow up with different relationships. He might form stronger bonds with his parents and be able to learn more about his family. Also, he could start training earlier and become an even more powerful ninja. With his knowledge from the future, he could also help the village in new ways, like coming up with better strategies against enemies.