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.
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.
Boruto could be developed by showing his growth in responsibility. For example, in the fanfic, he could take on the task of teaching Himawari a new jutsu. This would not only show his skills but also his patience. Himawari, on the other hand, could be developed by showing her becoming more independent. Maybe she decides to go on a small mission on her own to prove herself to Boruto.
In this kind of fanfiction, Naruto might teach Kawaki about the values of the Hidden Leaf Village. Maybe he'd show Kawaki how to control his powers better. For example, Naruto could take Kawaki on training sessions similar to the ones he had with Jiraiya.
One great idea for a Boruto and Himawari fanfic is a story where they go on a journey outside the village. They might encounter new types of ninjas or strange creatures. Himawari could be the one to sense danger with her Byakugan first, and Boruto would be ready to fight. It would show their growth as ninjas and also their bond as siblings.
Kawaki might resist Naruto's teachings at first. Since he has his own way of doing things, adapting to Naruto's values and training methods could be difficult. Also, the village might not fully accept Kawaki right away, and Naruto would have to fight for Kawaki to be accepted, which is a big challenge.
One great fanfic could be about Himawari and Sakurako going on a school adventure together. Maybe they are paired up for a project and during the process, they discover new things about each other. For example, they find out they both have a secret love for a certain type of music. This could lead to them bonding over it and becoming even closer friends.