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.
Often, Naruto is sent back to a time when he can prevent a tragic outcome or gain crucial knowledge. It could be to the time before his village was attacked or when a key character made a fateful decision.
Jiraiya is often a popular character in 'Naruto sent back in time fanfiction'. Since Naruto has a strong bond with him in the original series, in these time - travel stories, Naruto can learn even more from Jiraiya in the past and they can have new adventures together. Kakashi is also favored as he can be a mentor to Naruto in a different way with Naruto's new perspective.
One interesting plotline could be Naruto using his future knowledge to prevent the Uchiha massacre. He might be able to change things in a way that doesn't lead to Sasuke's downward spiral. Another could be him training his younger self to be stronger faster, so that he can face the early threats in the Naruto world more effectively.
Well, they could change a lot of things. Maybe Naruto would prevent some of the tragedies that happened in the past. He could stop the Nine - Tails attack on the village, which would completely change his own life and the lives of many in Konoha.
If Naruto and Hinata went back in time in fanfiction, one possibility is that they could change the course of many events. For example, Naruto might be able to prevent some of the tragedies that occurred in his childhood. Hinata could also use her knowledge to help her clan earlier. They might also have the chance to influence the relationships between different characters in a positive way, like improving the relations between the Uchiha and the village.
In some fanfictions, Naruto might go back in time and try to save his parents. He could alter the events that led to their deaths. This would not only change his own life but also have a huge impact on the entire ninja world as his parents were very powerful ninja.
Danzo could still be a major antagonist. Even if Naruto travels back in time, Danzo's thirst for power and his underhanded methods would still pose a threat. Naruto would have to find a way to deal with him without causing too much chaos in the village.
Well, they could also change their own relationships. Maybe Naruto wouldn't be so alone as a child, and Ino could become a stronger ninja earlier. They might train together in ways they didn't before, using the knowledge of future jutsu and strategies. They could also influence the relationships between different ninja clans, like improving the relations between the Uzumaki and the Yamanaka clans.