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.
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.
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.
There could be many out there. You could also check Naruto - centered forums. Fans often share their favorite fanfictions there. Sometimes, they even create their own stories and might have written something along the lines of Naruto as an old man traveling to the past. Another option is to look for Naruto fanfiction groups on social media platforms. They might be able to point you in the right direction.
His friends like Sakura and Shikamaru would be affected too. With Naruto's changes, their relationships with him and with each other could be very different. For example, if Naruto changes the course of events, Sakura might not be as fixated on Sasuke in the same way. And Shikamaru might find himself in different strategic situations due to Naruto's interference.
His character might become more cautious. Since he already knows what the future holds, he would be more careful in his actions to avoid making the same mistakes. For example, he might not be as impulsive when facing enemies.
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.