First of all, they would be astounded by the creativity of the fanfiction writers. Regarding Naruto's future in it, if he is shown as a great leader, they would beam with pride. But if there are aspects that distort his character, like making him overly cruel or something against his nature, they would be disappointed. And if he has to face some new and terrible enemies, they would feel the urge to protect him even though they are just reading a fictional account.
Minato and Kushina would likely have a complex reaction. They would be proud of Naruto's heroic deeds in fanfiction. But if they saw him being in danger or having a hard time, they would be worried. Also, if there are parts where Naruto's values seem different from what they taught him, they might be a bit confused.
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.
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.
Naruto would probably have a range of reactions. He could be excited to see new adventures that people have written for him. But if the fanfiction was inaccurate about his character, like making him cowardly when he's known for his bravery, he would likely be a bit miffed. He might also be curious about the new relationships with other characters that the fanfiction presents.
Some of them might be shocked. Tony Stark, for instance, might immediately start thinking of ways to use technology to change what he saw. Steve Rogers would probably be more concerned about the moral implications of interfering with the future.
They might be confused at first. Fanfiction can be wild and different from their real experiences.
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.
Sasuke would be deeply affected. He and Naruto had a complex relationship, from being rivals to friends. He would likely feel a sense of guilt, as if he should have been there to protect Naruto. Sakura would be heartbroken. She had always cared for Naruto, and seeing him die in such a tragic way in front of his parents would make her feel helpless. His parents, on the other hand, would be in utter despair. His father might feel like he failed as a father again, not being able to protect his son this time either.
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.