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.
Perhaps Anko rejects Naruto's feelings in a very harsh way in the fanfiction. She might say mean things about him not being good enough or worthy of her love. This kind of rejection can be really heart - breaking for Naruto who is often shown as someone with a big heart and strong emotions.
There could be many unknown authors out there writing fanfictions. It's hard to say specifically who the author of this one would be without more research. It might be some Naruto - loving fan who wanted to explore a more tragic and angsty storyline with these characters.
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 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.
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.
Well, it could be that Annabeth betrays Percy's trust. Maybe she lies to him about something really important, like her true feelings or her actions. This lie could be the start of breaking his heart.
Well, it could be that Artemis, in the fanfiction, has to make a choice that goes against Percy's feelings. Maybe she has some divine duty that forces her to break his heart. For example, she might have to sacrifice their relationship to protect her hunters or to maintain some sort of balance in the mythological world.