Sasuke would likely be enraged. Naruto is his rival and also, in a way, his friend. He would immediately set out to find Naruto and confront Itachi. Sasuke has a strong sense of pride and would not let his brother get away with this.
Sasuke could also be confused. He has always had a mix of emotions towards Itachi, including love, hate, and envy. Being kidnapped by him would add another layer of confusion. He might wonder if Itachi has some new plan or if he's finally going to reveal some long - hidden truth. So, he might hold back on his full - fledged resistance and wait to see what Itachi has to say.
Sasuke would probably be very confused at first. He has spent so much time hating Itachi for what he did to the Uchiha clan. Seeing Naruto befriend Itachi would make him question his own long - held beliefs. He might initially be angry with Naruto for seemingly siding with his enemy.
Sakura would probably be initially terrified. She would try to use her medical ninjutsu skills to defend herself or at least slow Itachi down. If she realizes she can't overpower him physically, she might try to talk to him, asking him why he's doing this.
There would be other fans who might be intrigued. They like to explore different 'what - if' scenarios in fanfiction. They would see it as a creative way to explore the characters in a new light, even if it goes against the canon. However, they would also be aware that it's just a fictional alternative and not the real Naruto story.
I think young Sasuke would have a very intense reaction. He was so consumed by his hatred for Itachi at that young age. A Sasuke vs Itachi fanfiction would be like a trigger for him. He would be eager to see how the fictional fight is portrayed. If Sasuke in the fanfiction is shown as strong, it might fuel his own ego and make him strive even harder to become powerful. However, if the fanfiction shows Itachi as superior, it could enrage him. He might feel that it undermines his own goal of defeating Itachi and he might even want to find the writer to correct their 'wrong' view.
Well, Naruto might be furious. He's always been protective of Sakura. He'd probably drop everything he's doing and go after Sasuke to rescue Sakura. Naruto has a strong sense of justice and loyalty, especially towards his friends. And Sakura is one of his closest friends, so he won't stand by and let her be in danger.
Sakura would probably be initially shocked and angry. She has always cared about Sasuke, and this betrayal would hit her hard. But she is also a strong - willed ninja. So, she would quickly gather her wits and start looking for ways to escape. She might try to use her knowledge of Sasuke's fighting style against him if she gets the chance.
At first, Naruto would be stunned. But being Naruto, he'd quickly gather his wits. He'd probably joke around a bit to lighten the mood and try to get Hinata to open up. When he realizes she's serious, he'd start to analyze the situation. He'd think about what could have made Hinata do this and try to find a solution that would not only free him but also help Hinata if she's in some sort of trouble. He might also think about how this would affect their friends and the village as a whole.
Naruto would definitely fight back. Despite being kidnapped, she has a strong will and a lot of chakra. She would use her Shadow Clone Jutsu to create distractions and try to break free from her captors. And she would also be yelling at them, trying to get them to see the error of their ways.
Itachi would probably be very calm and composed. He might see Sarada as a symbol of the new generation of ninjas in the village. He could be gentle with her, perhaps sensing her potential and her desire to grow as a ninja.