One challenge could be the difference in their personalities. Haise is more reserved and Mutsuki can be quite intense. This could lead to conflicts within their partnership, like misunderstandings during missions. Another challenge might be their loyalty to the CCG. If they start to question the CCG's motives, they would have to decide whether to stay loyal or follow their own moral compass. For example, if the CCG orders them to do something unethical.
One idea could be a story where Haise Sasaki and Mutsuki are on a mission together and they encounter a powerful ghoul. They have to rely on each other's skills to survive. Haise uses his detective skills to analyze the ghoul's behavior while Mutsuki uses his combat abilities. Another plot could be a character - development - focused story where Mutsuki is having doubts about his place in the CCG and Haise helps him through it, sharing his own experiences of identity struggle.
Yes, there are likely fanfictions out there with such a concept. The Naruto fandom is vast and creative, and crossovers with historical or other fictional characters like Sasaki Kojiro are not unheard of. Some fans might be intrigued by the idea of Naruto having some sort of connection or admiration for Sasaki Kojiro and create stories based on it.
Well, I've heard of a 'Hide x Haise' fanfic named 'Hide's Hidden Feelings'. In this one, the author beautifully portrays how Hide might be secretly in love with Haise and the internal struggle he goes through to keep their friendship intact while dealing with his emotions. Then there's 'Haise's Refuge in Hide' where Haise finds solace and comfort in Hide during difficult times, which is really heart - warming to read.
Hey, let me recommend a Doujinshi novel by Go Soul to you. It's called Go Soul: Onmyoji System." The novel was mainly about a person named Light who transmigrated to the world of Go Soul and met his childhood sweetheart, Fujiwara Sasaki. The two of them went through many hardships together and finally got together. The author of this book was Jun Wushang, and the quality was quite good! I hope you like my recommendation. Muah ~