In the fanfiction, it could be that she is betrayed by an authority figure in the guild. They might deny her a crucial mission or opportunity based on false accusations. This would lead to her feeling undervalued and betrayed, and she may then set out on a journey to prove herself independently.
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.
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.
Well, one possible plot could be that Natsu is betrayed in his own world and somehow ends up in the DxD Highschool universe. Maybe he has to deal with the different power dynamics and politics there while also trying to figure out who betrayed him in his original world. He might form new alliances with the characters in DxD Highschool and use his unique abilities to survive and seek revenge for his betrayal.
One possible reason could be a misunderstanding within the guild. Maybe someone spreads false rumors about Natsu, leading others to believe he's done something wrong. Another reason might be a power struggle. If there are those in Fairy Tail who envy Natsu's strength and popularity, they could plot against him to get him out of the way.
One possible plot could be that Natsu, being in the DxD High School setting, is betrayed by someone he considered a close friend. Maybe this friend reveals Natsu's secrets to the enemies in the DxD world. As a result, Natsu has to face new challenges alone, all while trying to figure out how to deal with the betrayal and regain his trust in others.
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.
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.
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.
Natsu could also be deeply hurt. He values his friends highly in Fairy Tail. If betrayed, he might isolate himself, doubting his own judgment in trusting people. This could lead to a period of self - discovery where he tries to figure out how to trust again. For example, he might recall the times when his friends in Fairy Tail were truly there for him and use those memories to heal and grow stronger in the face of this betrayal.
One possible reason could be a misunderstanding within the guild. Maybe some false information was spread about them, leading others in Fairy Tail to turn against them. Another reason might be a power struggle. If Natsu and Erza were becoming too powerful, some jealous members could have conspired to make it seem like they were betraying the guild. Or perhaps an outside force manipulated the situation to drive a wedge between them and the rest of Fairy Tail.
One possible plot could be that female Acnologia, being a powerful and feared being, initially sees Natsu as an enemy due to his association with the dragon slayers. But as they cross paths more often, she starts to notice his determination and the kindness in him. Eventually, they team up against a common greater evil, and during their adventures, a romantic bond slowly forms between them.