Shirou could have a range of reactions. He might be excited if the future in the fanfic seems positive and full of achievements. He could be worried if it shows him facing great difficulties or making bad decisions. And he might be amused at some of the more outlandish ideas in the fanfic.
Naruto would probably be very excited and want to immediately start making changes for the better. He has always been about hope and improvement.
He would probably be shocked at first. Just imagine seeing an older version of yourself. He might be a bit skeptical too, thinking it could be some kind of trick.
If Shirou reads about his future in a fanfic, he might initially be shocked. He could find out things about his relationships that he hadn't considered. For example, if the fanfic shows him in a relationship with a certain character that he currently has no feelings for, it could change how he views that character in his real life. On the other hand, if it shows a future where he fails at something important to him, it could either motivate him to avoid that fate or make him feel discouraged.
Harry might be a bit skeptical at first. He would question whether this is really his future self or some kind of trick. But as the future self starts to share some personal details or knowledge only Harry would know, he would start to believe. Then, he could become eager to learn from his future self, asking all kinds of questions about his future, his friends, and his role in the wizarding world.
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.
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.
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 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.
They might be intrigued. The future in fanfiction could be full of new concepts and relationships that they haven't considered before.
In the fanfic, if Peter's future involves him becoming a more powerful superhero, Thor might be excited to see another strong ally. He could offer to train Peter in combat. Bruce, on the other hand, would be concerned about how Peter's powers could affect his mental state. He might offer to help Peter understand the scientific aspects of his powers. Clint would probably be the one cracking jokes, but also secretly be in awe of Peter's growth and potential.