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.
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.
Naruto might react with a sense of responsibility. If he saw a future where Konoha was in danger, he would feel it was his duty to do something about it. He could start by gathering information from the future vision, like the weaknesses of the enemy. Then, he would share this knowledge with his comrades and come up with a plan to protect the village.
There could be a character who has a disability in the ninja world, like being partially blind but having an amazing sense of hearing or smell. Naruto would see their potential and include them in the team. Another member could be a former criminal's child who is trying to make a name for themselves in a positive way. And there could be a super - intelligent ninja who is great at strategy but lacks in practical combat experience, and Naruto would help them gain that experience.
Some characters might be shocked. For example, Naruto himself would probably be really excited to see how his world is imagined in the future. He'd be happy to see that his story has inspired so many different ideas. Sasuke, on the other hand, might be a bit more skeptical. He'd analyze the fanfiction carefully, looking for any inaccuracies or misinterpretations of his character or the ninja world's values.
He would likely be very curious. Naruto has always been eager to learn and explore new things, so seeing future fanfiction would be like opening a door to a new world of possibilities for him.
Sasuke would initially be shocked. He's used to seeing Naruto as this energetic and somewhat indestructible force. But then he'd quickly shift to a more protective stance. He might cover Naruto while waiting for help. Sakura would be in a state of panic. She'd be running around trying to find someone to help, like a medical - ninja. Iruka would be the most level - headed. He'd assess the situation and if it was serious, he'd take Naruto out of the exam area to get proper medical treatment while also making sure Naruto doesn't feel like a failure for getting hurt.
At first, Naruto could be a bit skeptical. He might think it's some kind of trick or illusion. But once he realizes it's really his future self, he would be excited. He would want to know about his future family, if he has one. He'd also be interested in the state of the ninja world in the future. His reaction would be a mix of excitement, curiosity, and a determination to make sure his future turns out as good as his future self's seems to be.