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.
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.
Sure. Itachi's saving of Sasuke was a crucial moment in the manga. It was tied to his hidden motives and sacrifices for the greater good of the ninja world and Sasuke's future.
A possible fanfic could focus on a parallel universe where Itachi and Sasuke are not bound by the same clan conflicts. In this universe, they could meet in a more peaceful setting, like a small village. Their relationship could develop gradually as they discover shared interests. For example, they might both be interested in learning about ancient ninja arts, and through this common ground, a yaoi - inspired relationship could start to form.
One common theme is the struggle against family and clan expectations. In the Naruto universe, the Uchiha clan has a strict code. Sasuke being in love with Itachi goes against the normal family dynamic. Another theme is self - discovery. Sasuke has to come to terms with his feelings which are not only unorthodox but also might make him question his identity.
In fanfiction, Sasuke and Itachi's brotherly love is often depicted in very complex ways. Some stories focus on Sasuke's journey to understand Itachi after his death. He discovers the truth behind Itachi's actions, which were all to protect him and the village. This realization deepens his love and respect for his brother.
Fanfiction.net is also a good place to search. However, the tagging system might not be as detailed as AO3. But if you dig a bit and look through the Naruto section, especially in the Itachi and Sasuke character - centric stories, you may come across such a fanfiction. Some writers might be more subtle about it and not have the kiss as an obvious part of the title, so you'll need to read the summaries too.
Some popular themes include the exploration of their brotherly bond at a younger age. It could be about Sasuke looking up to Itachi in a more innocent time before all the tragedy. Another theme might be their training together, with Sasuke eager to learn from his older brother. Also, the family dynamic within the Uchiha clan when they were younger can be a focus, like how Itachi's actions even then were subtly influenced by the clan's expectations.
One common theme could be the exploration of Sasuke's emotional turmoil. Since Itachi is abusing him in this fanfiction, Sasuke might be shown as confused, angry, and deeply hurt. Another theme could be the power dynamic between them. Itachi being the abuser might have a significant amount of power over Sasuke, which could lead to themes of helplessness on Sasuke's part. Also, themes of revenge might be present. Sasuke might be constantly thinking about getting back at Itachi for the abuse, which could drive the plot forward in the fanfiction.
One common theme is the struggle between love and power. Since Itachi is a vampire, he often has to balance his newfound power with his love for Sasuke. Another theme is the idea of forbidden love, as their relationship might be seen as taboo in the context of vampire and human (or in this case, another Uchiha).