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.
At first, they might be a bit suspicious of each other. Boruto has heard stories about Kurama but has never really interacted with him. Kurama, on the other hand, might be cautious as Boruto is Naruto's son and not sure if he can be trusted. But as they spend more time together, they could start to respect and rely on each other.
Perhaps in this fanfiction, Neji is brought back to life somehow. Boruto, being curious, approaches Neji. Neji then discovers how much the ninja world has changed since his time. Boruto could show Neji the new technology in the Hidden Leaf Village. Neji might be a bit shocked at first but then start to see the potential in these new things. They could also form an unlikely friendship, with Neji guiding Boruto in matters of honor and discipline, which are values Neji held dear.
Maybe in the fanfiction, Jiraiya is summoned by a new jutsu gone wrong. When he meets Boruto, he mistakes him for Naruto at first. As they interact, Jiraiya realizes Boruto's own unique struggles. He shares his wisdom about handling the expectations that come with being a ninja from a famous family, and Boruto learns a great deal from him.
One possibility is that Kurama senses a new threat approaching and decides to team up with Boruto. They could go on an adventure together, with Boruto having to learn how to work with the powerful and sometimes grumpy Kurama. Along the way, Boruto could learn more about the nature of chakra from Kurama, which would be a great development for his ninja skills.
In a 'Boruto meets Kurama' fanfic, Boruto might learn about Naruto's past with Kurama. Maybe Kurama would share some of his old battles with Boruto, giving Boruto a new perspective on his father's journey.
Another plotline might be that Kurama, having been with Naruto for so long, has some unfinished business related to the ninja world. He enlists Boruto's help to complete it. This could involve uncovering secrets from the past or righting some old wrongs. And along the way, Boruto would grow as a ninja and as a person.
In this fanfiction, Boruto might learn some unique ninja techniques from Jiraiya. Jiraiya could see the potential in Boruto and train him in his own unorthodox ways. Maybe Boruto would be introduced to Jiraiya's toad summoning, which could add a whole new dimension to his combat abilities.
Sasuke is very likely to be involved. He has a close relationship with Naruto and is also an important figure in Boruto's life. His skills and knowledge could be crucial in any adventure or situation that Naruto and Boruto find themselves in. Another character could be Hinata. As Naruto's wife, she would surely be concerned about any strange events involving Naruto. And for Boruto, she is his mother, so she would play a role in any family - related or village - threatening situations.
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.