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 have a special talent for forging alliances. They might use this skill to bring together different parts of Rias' harem that were previously at odds with each other. As they do this, they gradually become an integral part of the harem. Or perhaps the OC has a curse that they need to break, and Rias and her harem offer to help. In return, the OC offers their loyalty and unique skills to the harem, leading to many adventures together as they search for a way to break the curse.
A possible plot is that the OC is from a different supernatural realm and brings with them a whole new set of rules and abilities. This could lead to the OC teaching Rias and her harem new ways to use their powers, and also cause some confusion as they try to incorporate these new elements into their existing lifestyle. Also, the OC could be on a mission to find something that is hidden within Rias' territory, and in the process, they fall in love with her and the members of the harem while also having to deal with their own mission's requirements.
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. 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.
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 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 that the male OC has a unique power that Rias needs to protect her peerage. So they start working closely together, and slowly feelings develop. For example, the OC might be able to sense impending danger to the group in a way no one else can.
One idea could be a story where Issei and Rias have to face a new, powerful enemy that targets their relationship. Maybe an ancient demon who sees their bond as a threat to the traditional demon hierarchy. Another plot could be a time - travel story where they go back in time and have to deal with past events that could change their present relationship. For example, they might have to stop an event that would have otherwise led to their separation.
A plot could involve the mute OC being a new transfer student in the school. Their muteness makes them an object of curiosity and some characters might initially misjudge them.
One idea could be the main character, as the harem queen, having to deal with rivalries among the members of the harem. Maybe there's a new girl who challenges her position and she has to prove herself through various magical trials.
Perhaps a plot where the members of Vali's harem start developing new and unique powers. They then have to train together to control these powers while also facing threats from other factions in the Highschool DxD universe. This would involve a lot of character development and relationship building within the harem.
Maybe the story could revolve around the main character accidentally hypnotizing the wrong person in the harem, which leads to a series of comical and challenging situations. He has to then find a way to reverse the effects without causing too much chaos among the group.