It could also be related to a prophecy or a pre - determined fate in the fanfic's plot. There might be some kind of dark prophecy that says Kawaki has to kill Naruto to prevent a greater evil. Or perhaps Kawaki is being controlled by an external force that has a vendetta against Naruto and uses Kawaki as a tool to carry out the murder.
A 'sasuke meets oc fanfiction' could be really interesting. It might explore new relationships and character developments. Usually, when Sasuke meets an original character (oc), it can lead to various plot twists. For example, the oc could be from a different ninja clan with unique abilities that Sasuke has never seen before. This could either make them allies or enemies depending on the circumstances. The fanfiction could also delve into Sasuke's reaction to this new character, whether he is suspicious, intrigued, or dismissive at first.
Sasuke might be initially surprised. He is used to the Sakura from his past, so seeing a different, more mature future Sakura could catch him off guard.
Sasuke would likely be aloof and guarded at first. He might see Batman as just another 'vigilante' like some of the shinobi in his world. Batman, on the other hand, would be cautious and try to figure out Sasuke's motives. Eventually, they could start sharing their combat techniques. Sasuke might show Batman how to use chakra in combat, and Batman could teach Sasuke about the importance of having a strong moral code.
In a Sarada meets past Sasuke fanfiction, it could be that Sarada has to keep her identity a secret at first. She might be on a mission in the past for some reason. As she observes past Sasuke, she could notice his skills and the way he interacts with others, like Naruto. This would give her a chance to understand the bond between her father and Naruto from a whole new perspective, which could be really interesting for the readers.
In the fanfiction, the interaction between Sarada and Sasuke's family could be filled with emotions. Maybe Sasuke's family has high expectations of Sarada, given her relation to Sasuke. This could create some tension at first, but as they get to know her better, they start to accept and love her.
One common theme is friendship. Writers often explore how Himawari and Kawaki, who come from different backgrounds, form a bond of friendship. Another theme is growth. They both grow as characters, learning from each other.
Kawaki makes his appearance in Boruto manga at a specific point, but I can't recall exactly. You might want to check the official manga sources for the precise chapter.