Perhaps the OC Queen is an exchange student from a different supernatural realm. She has to navigate the complex social and power structures in the Highschool DxD school while also hiding her true motives, which are actually to form an alliance between her realm and the ones in the story. This would lead to many interesting interactions and plot twists.
The OC Queen could be a reformer. She sees the flaws in the current system of the devil world in Highschool DxD and decides to change things. She starts by influencing the main characters with her progressive ideas, but faces a lot of opposition from the traditionalist factions. This could create a story full of political intrigue and character development.
One idea could be the main character, as the harem queen, having to deal with rivalries among the members of the harem. Maybe there's a new girl who challenges her position and she has to prove herself through various magical trials.
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 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.
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.
One idea could be that the Lucifuge OC has a hidden power that gets awakened during a major battle at the school. Maybe it's a power related to time manipulation which could turn the tide of the fight against a powerful enemy.
One idea could be an OC male who is a transfer student with a unique power, like the ability to manipulate shadows. He could use this power to protect his friends in the dangerous world of Highschool DxD.
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 OC demon with the ability to communicate with animals could be an interesting concept. In the Highschool DxD setting, this power could be used in various ways. For instance, they could gather information from the animals in the school's forest area about the movements of enemies. Or they could use animals to deliver secret messages between different groups without being detected by the more high - tech surveillance methods used by some of the factions.
One idea could be an OC who is a transfer student with a unique power that no one in the Highschool DxD universe has seen before. This power could attract the attention of the main characters and lead to various adventures and alliances.
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.