Most likely not. Usually, songs like 'Haru Haru' are inspired by imagination or general emotions rather than specific true stories. It's more about artistic expression and creating a certain mood or theme.
You can start by looking on fanfiction websites like Archive of Our Own. They usually have a wide range of fanfictions, including ones about Nana Shimura.
One popular 'bnha shimura tenko x shinsou fanfic' is 'Parallel Paths'. It explores their relationship in a post - war setting where they both have to deal with the aftermath of the battles. The author does a great job of showing their growth and how they come to understand each other.
The uniqueness of bnha shimura tenko x shinsou fanfic lies in the combination of their personalities. Shimura Tenko has a complex and often misunderstood past, while Shinsou has his own insecurities and determination. When paired together in a fanfic, it creates a dynamic where they can either support each other's growth or clash in interesting ways.
One idea could be that Danzo is transported to an alternate universe where he has to team up with a version of himself from that world to stop a common enemy. This would force him to confront his own actions and motives in a new light.
Naruto himself would be an interesting addition. His ideals of friendship and forgiveness could be in direct contrast to Danzo's more underhanded and secretive ways. It would create a lot of tension and potential for character growth for both of them.
Haru does love Legosi in the manga. It's evident in the way she cares for him, shows concern, and their shared experiences strengthen their bond, making it clear that her emotions towards him are those of love.