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.
Well, if Naruto and Juubi go back in time, the storyline could be completely rewritten. For example, Naruto might use his knowledge of future events to prevent some of the tragedies that occurred. He could stop certain villains from rising to power earlier. And with Juubi by his side, he would have an incredibly powerful ally. This could change the power dynamics in the ninja world from the very start.
For Naruto and Juubi going back in time in the fanfiction, their relationship with the village elders would be quite altered. The elders might be more cautious or even fearful of Naruto due to his association with Juubi. Meanwhile, Naruto's friends like Shikamaru might be both amazed and a bit worried. Amazed at the new power Naruto has and worried about the potential chaos it could bring. And Naruto could use Juubi's power to protect his friends more effectively from the start, strengthening their bonds.
Naruto's relationships would be different. He might be more of a mentor figure to his younger self or his friends. Since he has the power of the Juubi, he could offer a different kind of protection and guidance. He could also have a different view on his enemies, as he might be more inclined to try to change them rather than just defeat them, using the Juubi's power as a tool for persuasion.
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.
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 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.
Another interesting idea in this type of fanfiction is the character interactions. Naruto could interact with his parents when they were young. He could see how his father was as a young ninja, and how his mother was before she sacrificed herself. This would give him a deeper understanding of his family and his own identity. And with the Juubi's presence, these interactions could take on a whole new level of significance, as he might be able to use the Juubi's power to protect his family in the past, changing the course of his own family's history.