Well, it could be that Kurenai is somehow deceived into believing Naruto has done something really bad. Maybe she's tricked by an enemy who fakes evidence against Naruto. So, in her misunderstanding, she turns against him.
In this fanfiction, perhaps Kurenai is forced to betray Naruto due to some political situation in the Hidden Leaf Village. There could be a group within the village that wants Naruto out of the way for some reason, and they put pressure on Kurenai to side against him. This would lead to a very complex and emotional situation for both Naruto and Kurenai.
Another possibility is that there is no single main antagonist. It could be a combination of misunderstandings, village politics, and personal issues within Kurenai herself that lead to her betraying Naruto. For instance, the village elders might be pressuring Kurenai to keep Naruto in check, and at the same time, Kurenai has her own doubts about Naruto's power which could be misconstrued as a reason to betray him.
One possible plot could be that after the war, Naruto and Kurenai start to work closely together on rebuilding the village. As they spend more time together, they develop feelings for each other. Eventually, they get married and face challenges like dealing with the expectations of the village and Naruto's duties as a hero while also building their own family.
The plot might involve Kurenai seeing Naruto as a kindred spirit. Maybe she lost someone dear and sees in Naruto the same pain and determination. She adopts him and starts to teach him about the importance of family and bonds. Naruto could then grow up with a stronger sense of self - worth. He might also become more confident in his abilities earlier because he has Kurenai's guidance. This could change the way he approaches his training and his interactions with the likes of Sasuke and Sakura.
Well, perhaps in this fanfiction, Kurenai somehow gets involved in Naruto's life in a very maternal way due to some unforeseen circumstances. For example, she might be the only one who can understand a new power that Naruto has awakened. So, she decides to guide him like a mother would. She would teach him how to control this power, and also give him emotional support which he lacks so much. In the process, their bond grows stronger, and they face many challenges together, like fighting against new and powerful enemies who are after Naruto's power.
Well, it could involve Kurenai teaching Naruto some unique genjutsu skills. Maybe Naruto, who is known for his lack of finesse in genjutsu, would struggle at first but then gradually improve under Kurenai's strict but kind tutelage. He might also learn some new ways to sense chakra from her, which could enhance his combat abilities further.
In such a fanfiction, maybe Kurenai could help Naruto with his stealth skills. Since she's an expert in genjutsu, she might teach Naruto how to use genjutsu more effectively in battles, like against the Akatsuki members.
In such a fanfiction, the plot could involve Kurenai teaching Naruto her unique genjutsu skills early on. Naruto might grow up with a more refined and stealthy approach to combat. Also, he could inherit Kurenai's calm and collected demeanor, which would be a contrast to his usual rambunctious self. Kurenai could also help Naruto better understand his emotions, especially when it comes to dealing with loneliness, as she has her own experiences with it.
Perhaps Naruto owns Kurenai in the sense of protecting her. In the fanfiction, there could be a new and powerful enemy threatening the village. Naruto, with his great power, takes it upon himself to keep Kurenai safe. He stands in front of her in battles, and his actions and decisions end up dictating how Kurenai can move and act, thus 'owning' the relationship in terms of protection.
Perhaps there was a misunderstanding. Kurenai might have thought Naruto was involved in something bad. For example, she could have been misled to believe Naruto was working with a group of rogue ninjas. So she attacks him, and Naruto has to prove his innocence while also dealing with her attacks.
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.