Naruto might have a mixed reaction. He could laugh at the funny parts of the fanfiction. However, if the fanfiction misrepresents his friends or his village in a negative way, he might get angry. He's very protective of those he cares about. He may also be intrigued by the different versions of himself that are presented in the fanfiction, and might even start to think about how he can grow in those new directions.
Boruto might be in awe. He'd see Naruto's incredible journey filled with hardships and triumphs, and he'd probably feel a sense of pride in his father. He might also feel a bit guilty for sometimes not understanding Naruto's responsibilities as Hokage.
He might be shocked at first. Just imagine seeing things that haven't happened yet. It's quite a mind - blowing experience.
He might be surprised. He's used to his own life and adventures, and seeing it twisted and turned in fanfiction could be unexpected.
Sasuke would likely act like he doesn't care much. But secretly, he might be a bit intrigued as to why Naruto is into such things. He might think it's a waste of time or he could see it as another way Naruto is different from him.
Naruto would be very curious. He'd want to understand what's going on. Once he figures out that there's a big battle between different factions, he'd try to find a peaceful solution. He might start by talking to the leaders, using his charm and persuasion skills. If that doesn't work, he'd use his ninja powers to protect the innocent. He's not one to just stand by when there's injustice.
Naruto's perspective could expand in terms of power usage. In the BNHA fanfiction, there are so many unique quirks and how they are used in different situations. He might start to view his own chakra and ninja techniques in a new light. He could think about ways to combine different techniques in a more creative way, similar to how the characters in BNHA use their quirks in combination. He may also become more aware of the importance of public perception, as heroes in BNHA are very much in the public eye, which is different from the ninja world where secrecy is often key.
He would probably be very confused at first. Naruto has always trusted Tsunade, so the betrayal would come as a huge shock.
Naruto would probably be very surprised. He has heard about Rin from Obito and Kakashi. So when he actually meets her, he might be at a loss for words at first.
He would be heartbroken. Naruto values friendship above almost everything. At first, he might not believe it and try to reason with his friends.
At first, Naruto's eyes would widen in surprise. He'd pick up the fanfiction, not really knowing what he's getting into. As he starts to read, he'd be constantly muttering things like 'What the heck?' and 'This is so strange!'. He might try to find someone to explain it to him, but if no one could, he'd likely just shrug it off as something he doesn't need to understand.