One idea could be that the OC has a special power that attracts Natsu and the other harem members. For example, the OC can control a unique type of magic that is crucial in saving the world, and Natsu is drawn to her courage. Another plot could involve the OC being from a different dimension and Natsu has to help her adapt to his world while falling in love. And perhaps there could be a rival in the harem who tries to drive the OC away from Natsu.
Lucy could be a likely member. She has a close relationship with Natsu already in the original story. Another one could be Erza. Her strong and independent nature might be attracted to Natsu's new godlike power. And Wendy, being young and impressionable, might also be part of the harem as she looks up to Natsu.
Give them real - life emotions. For example, if the OC is shy, show how she blushes when Natsu talks to her. And for Natsu, show his insecurities sometimes, like when he doubts his strength in front of the other harem members. Also, let the characters make mistakes. The OC could accidentally use her magic in the wrong way and cause a small problem, which makes her more human.
Well, Mirajane could be part of the harem. She has a motherly side and would want to comfort Natsu if he was betrayed. Levy is also smart and resourceful, and she could use her skills to help Natsu out. Juvia, with her obsessive love for Gray, might transfer some of that intense loyalty to Natsu if she saw him in a desperate situation after being betrayed.
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.
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 great 'Fairy Tail Natsu x Harem' fanfiction is 'A New Adventure'. It has an interesting plot where Natsu finds himself in various unexpected situations with the harem members. The character development is well - done, especially how Natsu's relationships with each of the harem characters evolve over time.
One popular 'natsu x juvia harem fanfiction' is 'The Unlikely Bonds'. It has an interesting plot where Juvia finds herself in a harem - like situation with Natsu. The author does a great job of exploring the characters' emotions and the unique dynamics that come with such a pairing.
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.
One possible plotline could be Natsu's godlike powers attracting various characters to form a harem. Maybe his newfound powers make him a target as well as an object of admiration, leading to different relationships within the harem.