Naruto would likely be excited yet a bit scared. He'd be eager to make the future a better place but also worried about the bad things he might see. Sasuke would probably be more reserved, analyzing the future information carefully. Hinata would be shy about it but also determined to protect those she loves if she saw any danger in the future for them.
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.
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.
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.
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.
Sasuke, on the other hand, might be more conflicted. He has his own goals and ideals. If he saw a future that didn't align with what he wanted, he might first question the vision. But ultimately, he could also take steps to change it, especially if it involved the safety of his village or his loved ones like Naruto or Sakura. He has a complex personality, so his reaction would not be straightforward.
Naruto would be excited and ready to face whatever comes. He's always been positive and believing in his ability to change things. So, he'd see it as a challenge to make the future better.
Naruto would probably be very excited and want to immediately start making changes for the better. He has always been about hope and improvement.
They could be shocked. Some might not expect the wild ideas that fans come up with in fanfiction.