In this fanfiction, it could be that Erza has to break Naruto's heart for some greater good. For example, she might be from a different world (her Fairy Tail world) and know that their relationship is doomed because of the different universes they belong to. So, she coldly tells Naruto it can't work between them, shattering his hopes.
One possible plot could be that Erza is under some sort of spell or mind control. As a result, she acts out of character and breaks Naruto's heart. She might start dating someone Naruto hates or publicly humiliates Naruto, making him feel like he was wrong to have feelings for her in the first place.
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.
Another possibility is that there is no real 'antagonist' in the traditional sense. Instead, it could be the circumstances that act as the 'antagonist'. For example, the differences in their worlds, their duties, and the fact that they come from such different backgrounds could be what causes Erza to break Naruto's 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.
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.
Perhaps there is a prophecy that only Naruto and Erza together can fulfill. They start off as strangers but as they journey to fulfill the prophecy, they discover that they have a lot in common. They both have a strong sense of justice and a will to protect their friends. As they face numerous enemies and obstacles, their friendship turns into a deep respect for each other's abilities, and they become a formidable team.
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.
In this fanfiction, perhaps Erza is found by Naruto when she's injured. Naruto takes her back to Konoha. Since he has a big heart, he wants to give her a home. As she recovers, they start to bond. Erza might be amazed by the chakra system and Naruto could be intrigued by her magic. They could then go on adventures together, with Naruto protecting her like a father figure at first, and later they become equal partners.