A more detailed title could be 'In the World of Fanfiction: Fem Natsu's Journey After Betrayal'. This title not only mentions the main idea of fem natsu being betrayed but also implies that there will be a journey or development in the fanfiction that follows the betrayal.
Redemption is also a possible theme. Maybe the person who betrayed Fem Natsu realizes their mistake and tries to make it up to her. The fanfiction could then focus on whether Fem Natsu can forgive and allow for redemption, or if she is too hurt to ever trust that person again. It could also explore how the act of betrayal changes both characters involved in the long run.
It could be Gray. Although they are friends most of the time, in a fanfiction, there could be a plot twist where Gray's envy of Natsu's abilities gets the better of him, and he betrays Natsu. Maybe Gray thinks that by getting rid of Natsu, he can be the top mage in their guild.
She might become angry at first. Natsu is known for her fiery temper. So, she could lash out at the person who betrayed her, confronting them directly and demanding an explanation.
One common theme is misunderstanding. Natsu is often misjudged by his friends, leading to a sense of betrayal. For example, his actions might be misinterpreted due to some magic interference or the scheming of a villain.
Well, perhaps Erza was tricked into believing something false about Natsu. Maybe she was misled by some enemy who made it seem like Natsu was going to endanger the guild. So, in her sense of duty to protect the guild, she took actions that Natsu perceived as betrayal.
Perhaps Makarov, the guild master, could be a part of the story too. He might be the one who has to deal with the consequences of this betrayal within the guild. His wisdom and leadership would be put to the test as he tries to keep the guild together while Natsu and Erza are at odds.
Perhaps she is betrayed in a romantic sense. She falls for someone who then turns out to be using her for their own gain, like getting closer to powerful members of the guild through her. This would shatter her trust in love and make her more guarded in future relationships.
One possible scenario in a fanfiction of Fem Tsuna being betrayed could be that her closest friends turn against her because of a misunderstanding. Maybe there was some false information spread about her having some ulterior motives in the mafia world she's involved in. So, her friends, who used to trust her completely, start to distance themselves and even plot against her.