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.
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.
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.
Well, Boruto might feel a sense of pride seeing his father Naruto in action against Sasuke. He has heard so many stories about their battles, and now seeing it in a fanfiction version could make him realize the deep bond and rivalry between them. He might also analyze their fighting styles and try to incorporate some of their moves into his own combat techniques later on.
It's really hard to say. Naruto has the power of the Nine - Tails and years of combat experience. But Boruto is young and full of surprises. He has his own unique skills. I think it could be a tie. They both have strong wills and would not want to seriously harm each other.
One common theme is growth. Both Himawari and Kawaki are often depicted as growing both as individuals and in their relationship. Another theme is acceptance. Since they come from different backgrounds, fanfictions usually show how they accept each other's differences.
One great 'kawaki naruto fanfic' could be one where Kawaki has to learn a new jutsu from Naruto. In many fanfics, Kawaki is often portrayed as a complex character trying to fit into the Hidden Leaf Village. He might struggle with his past and Naruto could be the one to guide him through it.
In such a fanfiction, perhaps Kawaki is driven by some extreme ideology. Maybe he believes Naruto's way of peace is a weakness and decides to take him out to change the ninja world in a more radical way.