One common theme is brotherly love. Itachi's love for Sasuke despite the hardships they face is often explored. For example, in many fanfictions, Itachi sacrifices a lot to protect Sasuke.
The theme of family secrets also appears frequently. Since the Uchiha family has many secrets, fanfictions might explore how these secrets impact the relationship between little Sasuke and Itachi. For instance, how Itachi has to keep certain things from Sasuke to protect him, which later causes misunderstandings between them when Sasuke grows up.
Another theme is Sasuke's growth under Itachi's influence. We often see how Sasuke tries to emulate Itachi in his skills and behavior. It might be shown through their training sessions where Sasuke is constantly striving to be as good as his brother.
One common theme is redemption. Sasuke often sees Itachi in a new light and this helps him find a path towards redemption. He realizes that Itachi was not the monster he once thought and his love for Itachi becomes a part of his journey to make amends for his past actions.
One common theme is revenge. Sasuke's thirst for revenge against Itachi is often explored in great detail. Also, friendship, especially Naruto's friendship with Sasuke, is a big theme. Naruto's attempts to bring Sasuke back from the path of vengeance. Another theme is sacrifice, as Itachi's self - sacrificing actions for the greater good are often a central part of the stories.
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.
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).
One common theme is Sasuke's growth. Writers often explore how he developed his skills and mindset as a child. Another theme is his relationships, like his friendship or rivalry with other characters when he was young.
The theme of power and its consequences is also quite common. Sasuke, Naruto and Itachi all possess great power. Fanfictions might explore how they handle this power, whether they use it for good or are tempted to use it for their own purposes. For instance, Sasuke's pursuit of power for revenge in the original story can be re - examined in fanfictions. Also, family bonds are a significant theme. Itachi's actions were deeply related to his family, and fanfictions may delve deeper into how his relationship with Sasuke affects the overall story and how Naruto might interact with this family dynamic.
The exploration of their different power levels and how that impacts their relationship is also common. Sasuke's journey to become stronger and how Itachi's power was both a source of envy and admiration for Sasuke can be a central theme. In some fanfictions, they train together or face opponents together, which allows for a deeper understanding between them and the growth of their relationship outside of the main Naruto plot.
Reconciliation is a big theme. Itachi and Sasuke's relationship is often at the center of this. Naruto can be the catalyst in these stories, helping them to reconcile their differences. There are also themes of sacrifice, as Itachi has already sacrificed a lot in the original story, and fanfictions may expand on this concept in relation to all three characters.
One common theme is forgiveness. Sasuke has to forgive Itachi for all the pain he caused him, like killing their clan. In fanfiction, this process of forgiveness can be very detailed and emotional.