Including Kawaki in a Boruto x Himawari fanfiction can add a lot of depth. Since he is like a brother to Boruto, his relationship with Himawari can be explored. Maybe there could be some initial jealousy from Kawaki as Boruto and Himawari are so close. But over time, they all form a strong bond. Another good character is Mitsuki. His snake - like abilities and his loyalty to Boruto can bring new elements to the story, especially when it comes to protecting Himawari.
Well, obviously Naruto and Hinata should be included. Naruto as the father can give some wise advice to Boruto and Himawari. Hinata as the mother can show her love and care for them. Also, Shikadai could be a great addition. He is Boruto's friend and his calm and intelligent nature can contrast with Boruto's more impulsive side, and he can also interact with Himawari in an interesting way.
Definitely Sasuke. His relationship with Naruto is so important in the original story, and when it comes to a fanfiction with kid Naruto and Boruto, Sasuke can act as a mentor or a challenging rival for both of them. Another great character is Hinata. She has a special connection to Naruto, and in a fanfiction, she could play a role in guiding Boruto or providing support to kid Naruto in a new situation.
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.
The relationship can be shown as a strong sibling bond. For example, Boruto could be protective of Himawari when she is in danger during a mission. Or Himawari could be the one to cheer Boruto up when he is feeling down about his ninja training.
One idea could be a story where Himawari and Boruto go on a secret adventure together outside the village. They encounter a strange new jutsu and have to figure out how to deal with it, relying on their unique sibling bond.
Obviously Naruto and Boruto themselves are the most essential characters. But Sasuke can also be a great addition. His relationship with Naruto is well - known, and his skills and experience can add depth to the story. He could act as a mentor to Boruto as well.
One idea could be a story where Boruto and Himawari go on a secret adventure together outside of the village. They discover a hidden place full of strange creatures and have to work together to get back home safely.
You can begin by creating a unique setting. Say, a time when the village is under a special celebration. Himawari and Boruto could be involved in different activities related to the celebration. Describe the scenery, the people around them, and then focus on how they interact in this context. It gives a good base to develop their characters and relationship further in the fanfiction.
Well, perhaps a plot where Boruto and Himawari accidentally swap bodies due to a strange ninja jutsu gone wrong. This leads to humorous situations as they try to adapt to each other's lives and also understand each other better. Or it could be a story where they have to work together to protect the Hidden Leaf Village from a new and unexpected threat.
Splinter is another character that could be included. As their mentor, he can offer wisdom and guidance to Donatello and Leonardo. His influence can shape their relationship and how they deal with the challenges in the fanfiction.
The coach could be a great addition. He could be the one who gives Kazemaru and Endou special training sessions or advice that helps them grow as players and also strengthens their bond. And perhaps some of their opponents too. For instance, a rival team's star player who challenges Kazemaru and Endou both on and off the field, making them strive harder to improve.