Another plot idea is that the OC's Sharingan gets corrupted by the dark magic in the Highschool Dxd world. This causes the OC to struggle between the power - hungry nature of the corrupted Sharingan and their own moral compass. Their friends in the school then have to find a way to save the OC from the clutches of this dark power.
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.
A plot could revolve around the Sharingan OC being a target because their Sharingan is seen as a powerful tool. Devils might want to use it for their own gain, and angels might see it as a threat. The OC then has to fight to protect themselves and their friends. Also, the OC could use their Sharingan to unlock a hidden power within Issei, which changes the power balance in the DxD world. Another idea is that the OC comes from a parallel world where the Sharingan and DxD powers are combined, and they are on a mission to bring balance to the DxD world.
One plot idea could be a character from Naruto ending up in the Highschool DxD world and having the Rinne Sharingan. They could use its powers in the battles against the devils and fallen angels in the new world.
One idea could be to create an OC who has a Sharingan that has unique abilities. For example, instead of just the normal powers like predicting movements, it could have the ability to see through illusions created by devils in the Highschool DxD universe. The character could be a transfer student, perhaps from a hidden clan that has a connection to both the Naruto world (where Sharingan is from) and the Highschool DxD world. Their backstory could involve some sort of experiment or curse that led to them having the Sharingan.
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 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.
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.