Obviously, Naruto himself would be a main character. He's the one from the future, so the story would likely revolve around him. His future self might have different relationships with his old friends like Sasuke and Sakura. Sasuke could also be a main character as Naruto's relationship with him has always been a crucial part of the Naruto story.
One interesting plotline could be Naruto seeing how his actions in the past have shaped the future of the Hidden Leaf. For example, if he discovers that a decision he made led to a new alliance or a new type of jutsu being developed.
In a 'Naruto travel to the future fanfiction', Naruto would adapt through his strong sense of curiosity. He would be eager to explore every new thing he sees in the future. His resilience would also play a big role. If he encounters difficulties, like not understanding how a particular future - tech works, he won't give up. Instead, he'll keep trying until he figures it out. And his ninja training gives him discipline. He would use this discipline to study the new ways of life in the future, from the social norms to the technological advancements.
In Naruto future reaction fanfiction, characters might react to various things. For example, they could react to new jutsu developed in the future. They might be amazed at the power and creativity behind it. Maybe Naruto himself would be excited to see how the next generation has advanced the art of ninjutsu.
Obviously, Naruto himself is the most affected as he has to deal with the knowledge of the future. But his friends like Sasuke, Sakura, and Kakashi would also be deeply affected. If Naruto saw a future where Sasuke was in trouble, he would do everything to change it, which would impact Sasuke's life. Sakura, being Naruto's teammate and also caring about Sasuke, would be involved in any plan Naruto made. And Kakashi, as their sensei, would be there to guide and support them through the changes Naruto tries to make based on his future vision.
In 'naruto in the future fanfiction', perhaps Naruto has passed on his ninja way to a new generation. His son or disciple could be facing a new threat that combines old and new jutsu. Maybe they have to deal with a group that has found a way to use ancient, dark jutsu in a modern, technological world.
He might be really surprised. Naruto is used to his own world and the fanfiction could show things he never imagined. For example, if the fanfiction has his kids achieving great things in new ways, he'd be proud yet a bit shocked at how creative fans can be.
He might be shocked at how the village has developed. In some fanfictions, the village could have advanced technology. Naruto, being from a more traditional ninja world, would likely be both amazed and a bit confused at first.
He might be shocked at first. In fanfiction, the future could be full of unexpected changes. He could be excited to see how his friends and the village have developed. Maybe he'd be a bit sad if things didn't turn out as he hoped for his loved ones.
In the original Naruto Shippuden, the story was mainly centered around the present - day conflicts and the history of the ninja world. However, in this 'Watch the Future' fanfiction, it's likely to be more speculative. It could explore how the actions of the characters in the present lead to different future scenarios. There might be new technologies or forms of ninjutsu that are a result of the characters' progress in the story. The fanfiction may also focus more on the psychological and emotional growth of the characters as they face the uncertainties of the future, which was not always the main focus in the original series.