If Naruto characters read the future in a fanfic, it would be a really interesting concept. Naruto himself would be faced with the knowledge of things to come. He might see the fates of his friends and comrades. This could lead to him trying to train harder or change certain events. Sakura could also be affected. If she saw a future where she was still weak, she might strive to become stronger. And for the villains, like Pain, if he saw a future where his plans failed, he might come up with new and more devious strategies. All in all, it would completely disrupt the normal flow of events in the Naruto world within the fanfic.
They might be shocked. For example, Naruto himself, if he reads about future threats to the village in the fanfiction, he would probably start preparing immediately to protect his friends and the Hidden Leaf. It could change their present actions as they try to avoid bad futures or work towards good ones.
Well, Naruto could see the future of his friends. Maybe he'd find out that one of his friends was going to face a great personal struggle. He could then try to be there for them in the present and offer his support, or even try to change the future to prevent the bad things from happening.
Well, perhaps Naruto sees the future where his friends are in great danger. So he tries to change the course of events. He might start to make different decisions in his present to avoid the bad future he has witnessed. For example, he could try to resolve conflicts with certain characters earlier to prevent future wars.
In this fanfic, Naruto might see a future where his friends have achieved great things. For example, Sasuke could be leading the Uchiha clan to new prosperity. Naruto might also witness new threats to the Hidden Leaf Village and start preparing for them in the present.
In such a fanfic, Naruto might be shocked by the advanced technology in the future. He could see flying cars and buildings that reach the sky. His ninja skills would be put to the test in a whole new environment where physical strength might not be as dominant as it was in his time.
Well, Future Naruto might bring back some lost techniques. For example, if there were jutsu that were forgotten over time, he could reintroduce them. He could also have a different attitude towards the village, maybe more world - wise, and this could influence Minato's view on how to lead and protect the village.
They might see the destruction of their village and work hard to prevent it. For example, they could start training more rigorously or form new alliances.
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.
Well, some characters might try to change the future they read. For example, Ichigo might see a future where his friends are in danger and immediately start training harder to prevent it. He's always been the type to take action.
They might be really surprised. For example, Naruto himself would probably be shocked to see some of the wild interpretations of his adventures. Some characters might find the fanfiction funny, while others could be offended if they are portrayed in a negative or inaccurate way.