He could change a lot of things. Maybe he would prevent some tragedies that happened in the original story. For example, he might be able to save some characters who died. He could also use his knowledge from the future to make the ninja world a better place earlier, like teaching new jutsu to the younger versions of his friends.
In this fanfiction, Naruto might go back to the time when he was a child. He could use his future skills and wisdom to outwit his enemies from that time, like Orochimaru. He could also help his friends, like Sasuke, in a different way. Maybe he could prevent Sasuke from leaving the village in the first place. This would lead to a completely different story arc for both of them and for the whole Konoha village.
The main character is Naruto. In the past, he will surely encounter his parents. Minato and Kushina would be important characters in this fanfic. Also, Jiraiya, his mentor, would be a key figure. Since Naruto is going to the past, he might seek Jiraiya's guidance even more. And characters like Orochimaru would also be part of the story as Naruto may try to stop Orochimaru's evil plans earlier.
One interesting plot could be Naruto using his future knowledge to prevent some of the tragedies that happened in the original story. For example, he might be able to stop the Uchiha massacre. Another plot could be him training his younger self, leading to a much stronger Naruto earlier on. And perhaps he could also change the relationship between the different ninja villages, promoting more peace and cooperation from an earlier stage.
Perhaps the plot of this 'future Naruto goes to the past fanfic' is centered around Naruto's desire to reunite with his parents. In the future, he realizes that by going back in time, he could meet them when they were alive. So, he travels to the past. Once there, he not only gets to meet his parents but also tries to change some things in the past that would lead to a better future for the entire Hidden Leaf Village. He might use his knowledge of future threats to prepare the village earlier. He could also try to influence his younger self's relationships with other characters, like Sasuke. Maybe he tries to prevent Sasuke from leaving the village in the first place by showing him a vision of the future if he does leave. This would be a very emotional and action - packed plot, full of opportunities for character development and exciting twists.
Regarding his relationship with the village elders, Naruto might be able to influence them more. As a retired ninja who has seen the future of the village, he could persuade them to make different decisions, like being more open - minded about new techniques or more inclusive towards certain clans. This would reshape the power dynamics and relationships within the village.
The older characters such as Kakashi would be cautious. They would recognize Naruto but also be worried about the implications of someone from the future being in the present. They would want to know if his presence could change the future in a negative way. For example, they might fear that his actions could lead to the earlier awakening of a powerful enemy. As for the villains, they might see future Naruto as a threat and try to eliminate him as soon as possible. Orochimaru, for instance, would be very interested in Naruto's future powers and might try to capture him to study. And the new generation of ninja in the past would look up to future Naruto, seeing him as a symbol of what they could become.
One possible plotline could be Naruto using his knowledge from the future to prevent certain tragedies. For example, he might stop the death of a beloved character like Jiraiya.
Another plotline could be Naruto influencing the relationships between characters. He could try to bring Sasuke back to the village much earlier in the timeline, preventing a lot of the pain and conflict that occurred between them. Or he could help his parents, Minato and Kushina, in ways that would change the future of the entire Hidden Leaf village.
Obviously Naruto himself. Since he is the one going to the past, he would be the central character. His younger self would also be important as he would interact with his future self.