Well, another plot could be that the OC is an old friend of Sona from a different dimension. They are reunited in the High School DxD world. The OC has a special ability to see the future in fragments. Using this ability, they start to foresee some dangerous events that could endanger not only Sona but the entire school. So, they work together to prevent these disasters. This could involve a lot of strategy and some cool battles against various foes.
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.
A third plot idea might be Sona trying to modernize the traditions of the devil world within the context of the school. She could face opposition from more conservative devils, and the story could explore her struggle to bring about change while still maintaining her position and respect.
Perhaps Alex Black has a mysterious past that Sona is intrigued by. They could start off as acquaintances, but as time goes on, they form a deeper bond over shared interests in magic and the supernatural world of Highschool DxD.
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.