Well, you could make an OC that is a half - angel, half - devil. This would give the character a very complex backstory and internal conflict, especially in the world of Highschool DxD where the angels and devils have their own politics and power struggles. Also, for the OP part, this OC could have a special ability to combine angelic and demonic powers in a new and powerful way.
One plot idea could be an OC who is a reincarnated angel in the Highschool DxD world. The OC has to hide their true identity while trying to stop a secret group of devils from starting a war between the three factions. They could team up with Issei and his friends, but there would be constant suspicion from the devils around them.
One idea could be an OC with a unique sacred gear that has powers no one has seen before. For example, a sacred gear that can manipulate time on a small scale, allowing the OC to freeze time briefly during battles in Highschool Dxd's universe. This would add a whole new dynamic to the battles and interactions.
There's a fanfiction where the op oc gamer has the power to level up like in a game. In this story, they use their unique skills to navigate the complex world of Highschool DxD, making allies and facing off against powerful foes. It's really exciting as it combines the game - like mechanics with the supernatural elements of the series.
One popular 'Highschool DxD' OP OC fanfiction is 'The Red Dragon Emperor's New Path'. It offers a unique take on the story with an original character who has overpowered abilities and embarks on a different journey within the 'Highschool DxD' universe. The character development in this fanfiction is quite engaging as the OC has to deal with the complex politics and power struggles in the world of devils, angels, and fallen angels.
First, focus on character development. Give your OC a clear backstory, motivation, and personality. For example, if your OC is OP, there should be a reason for it, like a difficult past that led to their power. Second, know the Highschool DxD universe well. Use the existing lore and rules to make your story fit in. Third, create interesting conflicts. Maybe your OC has to deal with jealousy from other characters because of their power.
One idea could be a female OC who is a transfer student with a unique Sacred Gear that no one has seen before. This could lead to all kinds of adventures as the characters in the Highschool DxD world try to figure out what it is and how powerful it could be.
One idea could be that the loli OC is a new transfer student at Kuoh Academy. She has a special power that she's unaware of at first, like the ability to communicate with mythical creatures. As the story progresses, she gets involved with the main characters from 'Highschool DxD' and they help her discover and control her power.
Well, you could make your fallen angel OC a bit of an outcast even among other fallen angels. Maybe they have a different view on the war between angels and devils. For example, they could be trying to broker peace in secret. They might also have a special connection to a certain sacred object that was lost during their fall, and the story could revolve around finding it again. Also, giving them a distinct personality trait like being overly curious about human culture could lead to some fun and unique storylines.
Interaction with the existing cast. A good fanfiction will have the OC interacting meaningfully with characters like Issei, Rias, and others. This can include forming alliances, rivalries, or even romantic relationships. It adds depth to the story and shows how the OC fits into the already established world of 'Highschool DxD'. Well - written fight scenes are also important. Since the OC is overpowered, the battles should be exciting and show off their abilities in a cool way, like using their powers in creative combinations against strong opponents.
Well, you could have an OP OC who is a master of ancient magic. He could use this magic to outwit the devils, angels, and fallen angels in the DxD universe. Maybe he has a secret past that ties him to an ancient prophecy, and as he discovers more about it, he has to face powerful enemies. You can also focus on his relationships, like having a romantic interest with a character from the original series that changes the power dynamics between different factions.