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.
Well, it could be that the OC is a healer. Natsu often gets injured in his battles, and the OC is the one who takes care of him. This creates a bond between them. In the harem, the other characters might have different skills like combat or magic knowledge. They all compete in a friendly way for Natsu's attention. Maybe there's a big tournament where the OC has to prove herself not just as a healer but also as someone worthy of Natsu's love.
A plot could be centered around a quest. The OC and Natsu, along with the harem members, embark on a journey to find a lost magical artifact. During this quest, the OC and Natsu grow closer as they face various challenges together. The harem members each have their own motives and relationships with Natsu and the OC develop in different ways. For instance, some may start off as jealous but then come to respect the OC.
One idea could be that Natsu is betrayed by his closest friends in the guild, and in his time of despair, a group of female characters form a harem around him to help him heal and regain his confidence. For example, Lucy could be the first to reach out, using her celestial magic to show him there are still things to fight for. Another plot could involve Natsu being betrayed by a powerful dark mage who manipulates his guild against him. The harem, which might include Erza, Wendy, and others, then trains Natsu in secret to become stronger and take revenge on those who wronged him.
Perhaps the OC is from a different dimension where magic works in a strange way. Natsu, Mirajane, and Erza are intrigued by this. They team up to help the OC understand and control their magic in the world they are in now. Along the way, they encounter a powerful dark wizard who wants to use the OC's unique magic for evil. So, they have to fight together to protect the OC and the world from this threat.
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.
One idea could be that the OC is a powerful ninja from a far - off land. She comes to Konoha and catches the attention of Naruto and his friends. As she gets to know them better, she forms unique bonds with each of them, leading to a harem - like situation. For example, she might train with Naruto and they share their dreams of becoming Hokage and protecting the village. With Sasuke, she could be someone who understands his pain and tries to bring him back to the light. And with others like Shikamaru, she might engage in strategic battles and discussions, slowly building relationships.
One idea could be Natsu from Fairy Tail ending up in a different anime world like Naruto and having a harem of both Naruto and Fairy Tail characters. For example, Sakura could be attracted to his unique magic, while Lucy from Fairy Tail is also part of the harem, leading to some comical and action - packed situations as they all deal with different threats in the crossover world.
One idea could be the OC having a unique power that attracts the various characters in the harem. For example, if the OC can manipulate time in a certain way, it could lead to some really interesting situations. Maybe they use this power to save a member of the harem from a dangerous situation, which then makes that character develop strong feelings for the OC. Another plot idea is the OC being from a different dimension and having to adapt to the Highschool DxD world while building relationships with the harem members.
One idea could be an OC with a unique power that attracts the attention of multiple X - Men characters, leading to a harem - like situation. For example, an OC who can manipulate time on a small scale. This power could be both useful and dangerous, and different X - Men might have different reactions to it, some being intrigued and others being cautious.
One idea could be that the OC is a new fighter in the Mortal Kombat tournament. As they progress, they attract the attention of various characters from different realms who then form a harem around the OC. Maybe the OC has a unique power that intrigues them all.
A plot could involve the OC being a healer. Natsu gets injured often, so they spend a lot of time together during his recoveries. They start to fall in love secretly. Since healers are supposed to be neutral, a relationship with a rowdy mage like Natsu is frowned upon. So they have to keep it a secret. They might communicate through a special magic - infused flower that only they can understand the messages from. This adds an element of mystery and secrecy to their relationship.
One interesting plot idea could be Natsu going on a special quest with each member of his harem separately. This would allow for character development of both Natsu and the individual harem members. For example, when he's with Lucy, they could discover a new type of celestial spirit magic that only works in a specific, long - lost realm. Another idea is that an enemy targets Natsu's harem to get to him, so he has to protect them all while also dealing with his growing feelings for each of them.