A great plot idea is an OC who is a master of both magic and martial arts in an OP way. In the Highschool Dxd world, they could start a new club that combines these two elements, attracting characters from all over the school. This would lead to interesting interactions, rivalries, and perhaps even new alliances as they face off against the usual threats in the series.
One idea could be an OC with a unique sacred gear that has powers no one has seen before. For example, a sacred gear that can manipulate time on a small scale, allowing the OC to freeze time briefly during battles in Highschool Dxd's universe. This would add a whole new dynamic to the battles and interactions.
Another plot could involve an OP OC who is actually from a different dimension but gets pulled into the Highschool Dxd world. This OC could have different values and abilities compared to the native characters. They might try to find a way back home while getting involved in the various power struggles and romances in the Highschool Dxd setting.
One plot idea could be an OC who is a reincarnated angel in the Highschool DxD world. The OC has to hide their true identity while trying to stop a secret group of devils from starting a war between the three factions. They could team up with Issei and his friends, but there would be constant suspicion from the devils around them.
One idea could be Shirou using his projection magecraft in the Highschool DxD world to create unique weapons during battles against the devils or fallen angels. For example, he might project Excalibur to fend off a powerful devil attack. Another plot could involve Shirou forming an alliance with Issei and his group, teaching them about his different combat techniques and how to better strategize in fights. He could also have a romantic sub - plot with one of the female characters from Highschool DxD, like Rias, where his noble and self - sacrificing nature attracts her.
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 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.
One idea could be an OC demigod who has the power of a forgotten god. They could use this power to help the main characters in unexpected ways, like having the ability to manipulate time on a small scale during battles. Another plot could involve the OC demigod trying to find their place in the world of Highschool DxD, being half - god and half - something else. They might struggle with acceptance from both the devil and angel factions.
One idea could be an OC who discovers an ancient and forgotten form of Excalibur. This could lead to a power struggle among the various factions in the Highschool DxD universe as they all vie for control of this newfound power. The OC would have to navigate through political intrigue and dangerous battles to keep the Excalibur safe.
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 Devil who has a unique power related to time manipulation. For example, they can freeze time briefly in battles, giving them an edge. Another could be an OC Devil from a long - lost noble family trying to reclaim their family's honor and status within the Devil society. They would have to face many challenges and rivalries along the way.
An interesting plot could be an OC gamer who is known for their role - playing skills in the human world. When they enter the Highschool DxD world, they start to role - play as a powerful being. This act fools many of the characters at first, but as time goes on, they have to actually become strong to keep up the facade and end up changing the power dynamics in the DxD world.