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.
A common theme is protection. Itachi is often depicted as protecting baby Sasuke from various threats. Another theme is brotherly love. It shows their bond even when Sasuke is just a baby. Growth is also a theme, as we can see how Sasuke might grow up under Itachi's influence in the fanfiction.
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 is redemption. For example, Sasuke often has a journey of redeeming himself after his dark turn. Another theme is friendship, like how Naruto's unwavering friendship for Sasuke is a central part. Also, the idea of sacrifice, as seen with Itachi's great sacrifice for the village and his brother.
One common theme is redemption. For example, Itachi's actions are often explored in a way that he tries to atone for his past deeds. Naruto usually helps in this process as he believes in the good in people.
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 sacrifice also often appears. Itachi sacrificed a great deal for Sasuke and the village. Fanfics might expand on this, like showing how Sasuke comes to fully understand and appreciate Itachi's sacrifices, and how that understanding changes their relationship.