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 and Itachi's story begins with Itachi being an older brother who Sasuke looked up to. But then came the fateful night when Itachi killed the Uchiha clan. Sasuke was filled with hatred and vengeance. He left the village to become stronger. Itachi, on the other hand, joined a group called Akatsuki but was still secretly looking out for Sasuke. As Sasuke grew stronger, he finally faced Itachi in battle. Itachi used his last strength not only to fight Sasuke but also to transfer some of his powers to him. Later, Sasuke found out that Itachi was actually a hero who had made the ultimate sacrifice. He had to live as a villain in Sasuke's eyes to protect the village and his little brother. This revelation changed Sasuke's perspective on life and his goals.
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.
The key event was the Uchiha clan massacre by Itachi. Then Sasuke's determination to kill Itachi in revenge. Their final battle was also crucial. And finally, the revelation of Itachi's true motives.
Maybe it's about protecting Sakura. Itachi might think that Sasuke isn't the right person for Sakura and wants to protect her from Sasuke's darker side. And Sasuke, on the other hand, believes he can protect Sakura better than Itachi and that Itachi has no right to interfere.
One reason could be Naruto's power. In the Naruto universe, Naruto has great power, and both Itachi and Sasuke might see Naruto as a key to achieving certain goals. Itachi might want to protect Naruto because he sees Naruto as a force for good in the future of the ninja world. Sasuke, on the other hand, may view Naruto as a rival in terms of power and influence, and in fanfiction, this could lead to a fight between them over Naruto.
In many fanfictions, Sakura might choose Sasuke. They have a long - established relationship, with Sasuke being someone she has loved since childhood. Her feelings for him are deep - rooted and often in fan - created stories, she remains loyal to those initial feelings.
Itachi. He was a highly skilled and intelligent shinobi. His Sharingan abilities were extremely powerful, and he had a lot of combat experience. He also had a strategic mind, which would give him an edge in a fight over Naruto.
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.