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.
The OC could be a reincarnation of an old ally of Lucifer. They start having strange dreams about their past life and gradually regain their old powers. Along the way, they have to deal with the politics and rivalries in the Highschool DxD world, all while trying to figure out their true place beside Lucifer.
One idea could be a new character joining the school who has a mysterious power that both the devils and angels are interested in. This new character could disrupt the existing power balance in the school.
A plot could be that the Akatsuki from Naruto are after a powerful artifact in the Highschool DxD world that is related to Lucifer. This would bring the two universes into conflict as the devils and other supernatural beings in Highschool DxD would not want the Akatsuki to get their hands on it. Naruto characters like Sasuke or Kakashi could be pitted against the devils like Issei or Rias. And Lucifer could be in the middle, either trying to stop the Akatsuki for his own reasons or using them to gain more power in his own world.
A plot idea could be Vali being sent on a secret mission by the Devils. He has to infiltrate an enemy stronghold that is rumored to be developing a weapon that could destroy the balance between the three factions. Along the way, he teams up with some unexpected allies, including some from the Angel faction who also don't want the balance to be disrupted.
One idea could be that Lucifer has a secret mission on Earth related to the events in Highschool DxD. He could be observing the main characters to see how they are handling the supernatural situations, and perhaps he has a greater plan in store for them. Maybe he's looking for a new heir or a special power that could be hidden among the students.
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.