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.
Well, you could make an OC that is a half - angel, half - devil. This would give the character a very complex backstory and internal conflict, especially in the world of Highschool DxD where the angels and devils have their own politics and power struggles. Also, for the OP part, this OC could have a special ability to combine angelic and demonic powers in a new and powerful way.
Another plot could involve the OC being sent by an unknown force to either help or hinder Lucifer. They initially seem like an enemy but as the story progresses, it's revealed that they are actually on the same side. They could team up to fight against a common threat to the devil world, like an ancient evil that is starting to rise again.
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 is a gamer and discovers a hidden power within the Highschool DxD universe while playing a virtual reality game related to it. This power could be something unique like the ability to control shadows in the DxD world.
One idea could be an OC who is a hybrid of a devil and a fallen angel. The story could revolve around their struggle to find a place in both worlds. They might face discrimination from pure - blooded devils and fallen angels alike, but also discover unique powers that come with their hybrid nature.
One interesting plot idea could be an OC with a Sharingan who transfers to the Highschool Dxd world. They could use their Sharingan abilities to detect the devils' true forms and powers. For example, during a Rating Game, the OC could analyze the opponents' strategies in an instant using the Sharingan's precognitive abilities.
One idea could be a Neko OC who has a unique power, like the ability to control shadows. She could use this power to protect the main characters in secret, and as the story progresses, she gradually reveals herself and her motives. Another plot could involve a Neko OC from a different dimension who gets lost in the Highschool DxD world and has to find a way back while getting involved in the local supernatural affairs. Maybe she forms an unexpected alliance with a group of devils or angels.
One idea could be an OC who is an exorcist with a special power to see the true forms of devils. They could start off as an enemy to the devils in the Highschool DxD world but then gradually form alliances due to a common threat. For example, a new, more powerful and evil force emerges that targets both exorcists and devils.