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.
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 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 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.
He might feel anger. Since he was so set on revenge against Itachi, any portrayal that didn't match his view of himself as the victor could make him angry.
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.
Sakura might be heartbroken at first. She has always been in love with Sasuke and to find out he's cheating would shatter her world. She could become withdrawn and focus more on her medical ninjutsu as a way to cope.
He'd probably be very intrigued. Percy has a penchant for adventure and new things, and this fanfiction would be something completely different from what he's used to.
In this kind of fanfiction, Itachi might use his powerful Sharingan abilities. He could perhaps create illusions to mislead Orochimaru and his minions, then quickly snatch Sasuke away. For example, he could make Orochimaru think that Sasuke is in a completely different location while he secretly approaches and rescues him.
In many fanfictions, Sasuke starts as a character full of anger and a thirst for revenge. But in the context of a relationship with Itachi, he begins to soften. He starts to see Itachi in a different light, beyond just the person who killed their clan. Itachi, on the other hand, is often portrayed as a complex figure with many secrets. In these fanfictions, his love for Sasuke makes him more vulnerable and human. For example, in some stories, he reveals his true motives to Sasuke, which changes their dynamic.