Maybe the OC has a different view on the guild's rules compared to Natsu. If the OC voices her opinions strongly and Natsu starts to question his own loyalty to Fairy Tail because of it, it could be a factor in his leaving. For instance, if the OC believes the guild should be more involved in outside conflicts while Natsu is more of a 'let's just protect our own' kind of person, this difference could drive a wedge between him and the guild.
The OC could have accidentally said or done something that made Natsu feel unwanted or unappreciated in Fairy Tail. For example, if the OC is a new member and got more attention than Natsu in a certain situation, it might have bruised his ego and led to his decision to leave.
One possible storyline could be that the OC tries to find Natsu after he leaves Fairy Tail. The OC might have a special bond with Natsu and be determined to bring him back. They could go on a journey where they face various challenges and meet new characters along the way. Another idea is that the OC is actually the reason Natsu leaves Fairy Tail in the first place. Maybe there was a misunderstanding or a conflict between them that led to his departure, and the story could be about how they resolve it.
Gray would be shocked at first. He and Natsu have a long - standing rivalry - but also a deep friendship. He might initially be angry that Natsu would just leave like that, but then he would realize how much he'll miss him. He might also feel a sense of loss for the guild as a whole, since Natsu is such an important part of it.
Gray could be one. Since he and Natsu have a long - standing rivalry, Gray might try to bring Natsu back to Fairy Tail. He'd be conflicted between his annoyance with Natsu and his concern for his former guildmate.
The OC could be a time - traveler who ends up in the Fairy Tail world. She meets Natsu and tries to hide her true identity. But Natsu, being perceptive, notices something odd. They start to team up for various missions, and the OC has to use her knowledge of future events to help Natsu and Fairy Tail, all the while falling for Natsu's brash but kind personality.
In fanfictions, Natsu's anger often has a significant impact on his relationships. His anger towards enemies unites him more closely with his Fairy Tail comrades. They see his passion and are inspired to fight alongside him. However, his anger can also create misunderstandings. For example, if he gets angry at a new member of the guild who made a mistake but was actually trying to help, it could make that person feel unwelcome at first. Overall, his anger is a double - edged sword in his relationships.
Create an engaging plot. Instead of following the typical 'Fairy Tail' storylines, come up with something fresh. Maybe involve a new villain or a mysterious place that Natsu and the OC have to explore. Add some twists and turns, like the OC having a secret connection to an old enemy of Fairy Tail that she doesn't know about until later in the story. This will keep readers hooked.
He could leave because of a misunderstanding. Maybe he overhears something wrong and thinks the guild doesn't trust him or value him anymore. So in a fit of anger and pride, he decides to leave.
If Natsu is a girl in this fanfiction, her character might change in terms of her relationships. For instance, she may be more self - conscious about her appearance compared to when she was male. Also, her interactions with male characters like Gray could shift. Instead of their typical rival - like banter, there could be a more complex emotional undertone, like a friendly - rivalry with a hint of romantic tension perhaps.
Suppose the OC is more introverted. In Natsu's body, they might have a hard time dealing with Natsu's boisterous friends at first. This could create some comical and also some tense moments in the story. But as time goes on, the OC might start to open up a bit more because of the influence of Natsu's friends, and in turn, they could also influence the guild members in new ways, like introducing quieter, more thought - out strategies during missions.
He might initially be shocked and confused. Natsu has always been so loyal to Fairy Tail, so this would be a huge blow to him.