Well, a cool plot could be that the little boy OC is actually a reincarnation of a powerful being from the past. He doesn't know this at first but starts having strange dreams and abilities. As he tries to figure out what's going on, he befriends some of the main characters in Highschool DxD. They initially don't take him seriously, but as his powers grow, they realize he might be a key factor in an upcoming big event, like a war between the devils and the angels.
A plot could involve the mute OC being a new transfer student in the school. Their muteness makes them an object of curiosity and some characters might initially misjudge them.
A really interesting plot would be if the male OC's time - stopping power has a limit. He discovers that every time he uses it, it drains his energy and he has to find ways to replenish it. This could lead to him having to make difficult choices about when and how often to use his power. Maybe he has to form an alliance with a magical being that can help him manage his power in exchange for some favors. And all this while, he's also dealing with the normal high school drama in the DxD universe like dealing with rivalries and making friends.
A good plot idea could be that the OC Dragon God has to hide their true identity in the high school. They are there to find a certain magical item that is hidden in the school. As they search, they get involved with the main characters and have to deal with the normal high school drama as well as the supernatural threats that come their way.
One idea could be an OC who is a dragon slayer but has a unique power that sets them apart. For example, they can absorb the powers of the dragons they slay instead of just killing them. This could lead to all kinds of new storylines as they become more powerful and are hunted by both good and bad forces in the Highschool DxD universe.
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.
First, give him a distinct personality trait. For example, he could be extremely curious, always asking questions about the strange world of Highschool DxD. Second, decide on his backstory. Maybe he has a sad past that makes him more empathetic towards others. Third, think about his relationships. He could look up to some of the older characters as role models.
Maybe the OC is a descendant of an ancient race that was once closely associated with the power of Divine Dividing. They come to the high school to find out more about their heritage and end up getting involved in the power struggles between the different factions in the DxD world. This could involve making alliances and enemies along the way.
One idea could be an OC who is a transfer student with a unique power, like the ability to manipulate time slightly. This catches the attention of the main characters in the Highschool DxD world. The harem could start to form as different characters are intrigued by the OC's power and mysterious background.
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.
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.