First, think about their backstory. For example, if they are a human, what was their life like before getting involved in the 'Highschool DxD' world? Maybe they had a difficult childhood which shaped their personality. Second, give them a unique skill or trait that makes them stand out but is still believable within the context of the series. It could be something like being an expert in martial arts which helps them in combat situations.
An OC (Original Character) in a Highschool DxD fanfiction having the Rinnegan could be an extremely powerful and unique addition. The Rinnegan, from the Naruto universe, brings a whole set of abilities like the Six Paths powers. In the context of Highschool DxD, this OC could use these powers in various ways. For example, the ability to see chakra could be translated into seeing the magical energy in the DxD world. They could use the gravitational powers of the Rinnegan to manipulate the environment during battles against devils, angels, or fallen angels. It would make for a very interesting and overpowered character in the Highschool DxD fanfiction setting.
The human OC can be developed by giving them unique abilities. For instance, they could have a special power that Rias doesn't have or understand. This would make them stand out and be a threat to Rias. Another way is through their backstory. A tragic or interesting backstory can make the readers sympathize with the OC and understand their motives for bashing Rias.
One common theme could be power dynamics. Since it's a Rias - bashing fanfiction, it might explore how a human OC challenges Rias' authority or power within the Highschool DxD universe. Another theme could be the exploration of different character relationships. The human OC may interact with other characters in ways that contrast with how Rias does, leading to new and interesting story developments. It could also deal with the concept of loyalty, as the OC might sway the loyalty of characters who were previously on Rias' side.
An OC who is a human detective investigating strange occurrences in the 'Highschool DxD' world. They could end up forming an unlikely alliance with the devils. This would bring a new perspective as they try to understand the complex relationships and power dynamics in the series.
The OC, as the Son of Sparda, could fit in by being a new and powerful entity. He might be initially seen as an outsider, but his powers would quickly draw attention. He could become involved in the power struggles between the different factions in Highschool DxD. For example, the devils might try to recruit him, while the angels might be wary of his presence. His unique abilities would also make him a valuable ally or a fearsome opponent. And in the high school setting, he could be a bit of an enigma to the other students, both human and supernatural.
One great fanfiction could be where the OC has a unique power that Akeno is initially intrigued by. For example, the OC might have the ability to control light in a different way than the devils in the Highschool DxD universe. This could lead to a series of adventures where Akeno and the OC team up to face various challenges. They could start off as just acquaintances but gradually develop a deeper bond as they fight side by side.
One possible aspect could be the unique abilities that Eve might possess in her reincarnated form. In Highschool DxD fanfiction, new characters often bring fresh powers. For example, Eve could have a special power related to light or healing, which would make her a valuable asset to the group. This could also lead to interesting storylines where she has to learn to control her powers while also dealing with the typical Highschool DxD chaos of devils, angels, and fallen angels.
To write a Highschool DxD OC Angel fanfiction, start with a strong concept for your angel. Are they a high - ranking angel with a lot of power or a newbie just learning the ropes? Next, think about their relationships. Who will they be friends with? Enemies? You can also add some unique abilities to your OC, like the ability to heal any wound or the power to manipulate light in a special way. As you write, keep the tone and style of the original series in mind to make it feel like it belongs in the Highschool DxD world.
One great Highschool DxD OC Angel fanfiction is 'The Angel's Ascent'. It has an interesting plot where the OC angel has to navigate the complex world of the DxD universe while hiding her true powers. It's full of action and some great character development.