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.
A plot could revolve around the OC being a sort of outcast in the school initially. But as the harem forms around them, they start to gain confidence. The OC could be a healer, and in the world of Highschool DxD where battles are common, their healing abilities become highly sought after. This leads to different characters, both good and bad, trying to get close to the OC, creating a complex web of relationships within the harem. The OC then has to navigate through these relationships while also dealing with their own insecurities.
The OC could be a reincarnation of a powerful being. In the Highschool DxD universe, this draws the attention of the harem. They might start off as friends, but as the OC starts to remember their past life and powers, the relationships become more romantic. For instance, the OC could have a connection to an ancient magic that is needed to protect the school or the entire world. The harem members then support the OC in their journey to unlock and control this power, all while their relationships develop on different levels.
One idea could be that the OC has a unique power that helps Rias and her harem in unexpected ways. For example, the OC might be able to sense danger before it arrives, allowing them to better prepare for battles against other factions. Another plot could involve the OC having a past connection to one of the members of Rias' harem, which creates some drama and tension as they navigate their new relationships within the harem. Maybe the OC is a long - lost relative or an old friend who has changed a great deal.
The OC could be a new transfer student with a mysterious past. As the story progresses, it's revealed that they are on a quest to find a powerful artifact that is somehow connected to the characters in the harem. This could lead to adventures together, and the building of deeper relationships within the harem.
One idea could be an OC who is a transfer student with a unique power. The harem could form as different characters in the Highschool DxD world are intrigued by this power and gradually fall for the OC. For example, the OC might have the ability to heal wounds instantly, which is very useful in the battles that often occur in the DxD world.
One idea could be an OC who is a transfer student with a unique power, like the ability to manipulate time slightly. This catches the attention of the main characters in the Highschool DxD world. The harem could start to form as different characters are intrigued by the OC's power and mysterious background.
One idea could be an OC who is a transfer student with a unique power that no one in the Highschool DxD universe has seen before. This power could attract the attention of the main characters and lead to various adventures and alliances.
Perhaps the OC is a skilled musician and Sona, who has an appreciation for the arts, meets the OC at a school concert. They start collaborating on music projects and their relationship blossoms from there. The OC could also have some sort of hidden past related to the supernatural world that Sona helps them uncover.
Maybe the OC is an artist who can create magical art. Akeno is fascinated by this art and the emotions it evokes. They could start collaborating on projects that are not only artistic but also have a deeper meaning in the context of the Highschool DxD universe. For instance, creating art that can temporarily shield them from attacks or art that can influence the minds of their enemies. Their relationship could grow as they explore the potential of the OC's art together.
Maybe the OC Sitri is from a different branch of the Sitri family that has been in hiding. Their appearance in the story could stir up old rivalries or create new alliances. They could bring with them ancient knowledge or artifacts that change the power dynamics in the Highschool DxD world.
One plot idea could be that the OC is a new transfer student at the school and has a special power that Koneko is initially suspicious of. As they are paired up for a school project, they start to understand each other better. The OC could be from a different supernatural group, and through their interactions, they help bridge the gap between their respective groups in the Highschool DxD universe.
One idea could be that the OC is a new student at the school and immediately catches Akeno's eye. They start off as friends but slowly develop deeper feelings as they go through various supernatural battles together. Another plot could involve the OC having a special power that Akeno is intrigued by, and they bond over exploring and controlling that power.