It could be that the girl is someone Naruto has known for a long time, like a childhood acquaintance. She might have always been jealous of Naruto's growing popularity and power. So, she decides to play with his feelings. She pretends to love him and then dumps him publicly, making Naruto feel humiliated and heartbroken.
She could be a kunoichi from a small, unknown village who comes to Konoha on a mission. Naruto, being the friendly and trusting person he is, befriends her. But she has a hidden agenda and uses Naruto's feelings for her own ends, breaking his heart in the process.
It could be that Hinata publicly humiliates Naruto. She might expose some of his deepest insecurities in front of the whole village. Since Naruto has always been striving for the village's acceptance, this betrayal from Hinata, who he loves, would be a huge blow to his heart.
In this fanfiction, it could be that Akeno chooses someone else over Naruto. She might publicly reject Naruto in a really humiliating way, like in front of all their friends and allies. This would definitely break Naruto's heart as he is a character who values relationships deeply. He would be heartbroken not just because of the loss of a romantic interest but also because of the feeling of being publicly shamed and rejected.
Well, it could be that Anko does something really cruel like betraying Naruto's trust. Maybe she lies to him about something he deeply cares about, like his dreams of becoming Hokage. And this lie shatters his hopes and thus breaks his heart.
Well, it could be that Hinata starts to ignore Naruto for some reason, like she falls in love with someone else. Maybe she gets influenced by some external force or false information that makes her believe Naruto isn't good for her. This sudden change in her behavior towards Naruto would break his heart as he might have deep feelings for her.
Well, it could be that Hinata might break Naruto's heart by choosing someone else over him. Maybe she realizes she has feelings for another character, like Sasuke perhaps. This would be a huge shock to Naruto as he has always seen Hinata as a special person in his life.
Perhaps Ino lies to Naruto about something really important, like leading him on with false hopes of a relationship. She could make him think she has feelings for him, but then suddenly reveals it was all a joke or part of a mission. This would shatter Naruto's heart because he's so sincere in his emotions. He'd feel used and betrayed.
Well, perhaps Erza might reject Naruto's feelings in a cruel way. Maybe she tells him she loves someone else, like Jellal. This would break Naruto's heart as he is used to being persistent in love, like his feelings for Sakura in the original series.
In a 'hana breaks naruto's heart fanfiction', perhaps Hana lies to Naruto about something really important to him. Like she might tell him she loves him but then be seen with another guy in a compromising situation. This could break Naruto's heart as he is usually a very loyal and loving character.
Well, perhaps the fanfiction could start with Naruto feeling that there is something wrong with his connection to the Kyuubi. He then embarks on a journey of self - discovery. Along the way, he meets some mysterious characters who give him clues on how to break the 'kyuubi lemon', which might refer to some sort of blockage or negative aspect in their relationship. As he progresses, he gradually overcomes these issues and in the end, he has a much stronger and more harmonious bond with the Kyuubi, and his power increases significantly. This plot could also involve some emotional turmoil for Naruto as he battles with his inner self and tries to understand the true nature of his relationship with the Kyuubi.