Well, it could be about Kurenai having some secret connection to Naruto as his sister. Maybe she discovers this truth and has to deal with the implications. For example, she might have to protect Naruto from new threats while also coming to terms with her new - found family relation.
One possible plot for a 'kurenai sister of naruto fanfiction' could be that Kurenai and Naruto are reunited as siblings. Kurenai then uses her position and skills to help Naruto gain more acceptance within the village. She could also help him better understand his own powers. Along the way, they face challenges together, like dealing with the Akatsuki who might see Naruto's new family connection as a way to get to him. Their bond as siblings grows stronger as they overcome these difficulties.