Happy could be an interesting character. He could be like a sidekick and a source of comic relief during their relationship journey. For example, he might accidentally interrupt their romantic moments or give Natsu really bad advice on how to court the OC at the beginning. Erza could also be important. She could be the one who gives the OC advice on how to handle Natsu's wild nature, as she has known him for a long time.
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.
In a 'natsu married oc fanfiction', perhaps the OC is a healer. Natsu gets seriously injured during a battle and the OC nurses him back to health. Natsu is so grateful and impressed by her kindness and skill that he starts to develop feelings for her. They get married and the story could focus on how Natsu has to adjust to having a partner who is more calm and collected compared to his fiery self. He might learn to be more patient and she might learn to be more adventurous from him.
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.
You could write an 'team natsu oc fanfiction' where the OC is from a different dimension and has different cultural norms. This OC could introduce new concepts and ways of thinking to Team Natsu. Maybe the OC's dimension has a different way of using magic, like combining it with technology, and Team Natsu has to adapt to this new form of magic while also teaching the OC about their own magic system.
The OC could be a character with a mysterious past. Maybe they have a hidden power that they are not fully aware of. Natsu, Mirajane, and Erza sense this mystery and are drawn to help the OC uncover their past. As they dig deeper, they find that the OC has a connection to an ancient magic that was thought to be lost. This discovery not only changes the OC's understanding of themselves but also has a significant impact on the relationships within the group.
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.
For Natsu, you can focus on his loyalty and protectiveness in the relationship. Since it's a secret, he might be more cautious and less impulsive than usual. For the OC, give her some unique traits. Maybe she's shy but has a hidden fiery side like Natsu. You can show her growth as she becomes more confident in the relationship.
One popular one might be 'Natsu's Unlikely Bond' where the OC has a unique magic that draws Natsu in. They start off as rivals but gradually develop feelings. Another could be 'A Mate for Natsu' which focuses on the OC being from a different realm and how they adjust to Natsu's world while falling in love with him. There's also 'Natsu and His OC Love' that shows their relationship growing through various adventures together.
In 'natsu x oc dragon slayer fanfiction', a frequently seen theme is the struggle against dark forces. Natsu and the OC team up to fight various villains and protect the world. Another common theme is family. In the Fairy Tail universe, family is important, and the OC may find a new family in the guild, especially with Natsu as a close friend or more. Additionally, the theme of growth, both emotional and in terms of magical power, is a staple in these fanfictions.