Sarada could also be a main character. She is strong - willed and intelligent. She could be investigating a case of a missing - nin who is suspected of being involved in some sort of conspiracy against the Uchiha clan. This would give her a chance to show her detective skills and her loyalty to her clan.
Kawaki could be involved. He has his own dark past and understanding of the hardships. He might encounter a missing - nin who has a similar past of being used and discarded. Kawaki could then have to decide whether to show mercy or follow the rules regarding missing - nin.
One idea could be a missing - nin who was once a friend of Boruto's and has some unique jutsu. This missing - nin could be on a quest to find a cure for a rare disease that his family has. Boruto, upon learning of this, has to decide whether to bring him back as a missing - nin or help him.
In a 'boruto shinachiku world fanfic', potential characters would include Shinachiku's family members for sure. Naruto, being his father, would play an important role. Hinata as his mother would be there to support and guide him. Boruto would be his cool older brother. Besides, characters like Konohamaru, who is now a teacher, could also be in the story. He could train Shinachiku and help him grow as a ninja. Also, the friends that Shinachiku makes on his own adventures, new characters created specifically for the fanfic could be part of it.
It could be the Kara organization. They've been shown to be interested in Boruto's Karma mark in the original series, so they might have finally made a move to abduct him in the fanfic.
Naruto would be devastated. He might blame himself for not being able to protect his son. Sasuke would be angry and would likely go on a hunt for the perpetrator. His friends like Sarada and Mitsuki would be heartbroken. Sarada might vow to become stronger to prevent such a tragedy from happening again.
Perhaps in this 'Boruto goes missing fanfic', Boruto is lured away by a mysterious figure who claims to have information about his future or his father Naruto's past. He follows this figure without thinking, only to find himself in a strange and dangerous place. His disappearance then sets off a chain of events in the village. People start to suspect that there might be a traitor among them who helped Boruto disappear. His teammates, like Sarada and Mitsuki, would then start their own investigation to find him, facing not only external threats but also internal doubts and suspicions within the village.
One potential storyline could be Naruto training Boruto to unlock a new form. This new form could be a combination of Boruto's unique abilities and Naruto's power. Another storyline might be Naruto teaching Boruto how to deal with the pressure of being his son and living up to the Uzumaki name.
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.
There could also be an ancient evil that was sealed away long ago but is now awakened due to the changes in the ninja world. This evil force could possess people or create chaos in the villages. Boruto and Naruto would have to figure out how to stop it, perhaps by seeking out old allies and learning forgotten jutsu.
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.
There could be a new villain in the fanfic. This villain might be after the power of the Nine - Tails that Boruto now wields. So this villain would be an important character driving the conflict in the story.