Often, there's a theme of challenging the status quo that Rias represents. In the original, she has a certain position and set of relationships. Fanfictions might bash her to create a new narrative where the OC can break free from the typical Rias - centric storylines. For example, if Rias is seen as too controlling in the original, the fanfic could have the OC resist her authority and form new alliances or ways of doing things.
A prevalent theme could also be the exploration of different character traits. The OC is used to contrast with Rias. If Rias is known for her pride, the OC in the bashing fanfiction might be humble. This contrast helps to drive the story forward and gives readers a new perspective on the Highschool DxD world that is not as Rias - focused.
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.
There could be the other members of Rias's peerage. In some fanfictions, they might be forced to choose sides between Rias and the OC. For example, Akeno might be torn between her loyalty to Rias and her growing respect for the OC's abilities.
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.
Well, one interesting aspect could be the exploration of different character dynamics without Rias being in a central positive role. It might show how other characters develop in a new light. For example, other devils or supernatural beings could step up and show their capabilities more prominently.
A common theme could also be her role as a leader. Fanfiction authors might write about her being a bad leader, making wrong decisions that lead to negative consequences for her group. This could be a way for the author to explore different leadership styles and show how things could go wrong under Rias's leadership.
One possible plotline could be that Naruto is transported to the Highschool DxD world and immediately notices Rias's overbearing attitude. He decides to oppose her and starts to gather allies among the other students and characters who are also fed up with her ways. As a result, a power struggle ensues between Naruto and Rias.
Power - up themes are also quite common. Issei often gets new powers or abilities, and Rias may play a role in that. For example, in some fanfictions, Rias helps Issei to master a new form of his Sacred Gear. This not only strengthens Issei but also deepens their bond as they train together. Another aspect is the exploration of the supernatural world in the context of their relationship. Since Highschool DxD has a rich supernatural setting, fanfictions may expand on this, like how Issei and Rias face new threats from the underworld or other dimensions together, which further cements their relationship and showcases their individual growth.
One common theme is power play. Since Rias is in a position of power, the fanfictions often explore how she wields that power in a femdom context.
Well, often in Highschool DxD Issei bashing fanfiction, a common theme is power redistribution. Since Issei is a central figure in the original in terms of power - up and his role in battles, these fanfictions might shift the focus to other characters getting stronger or more important in different ways. It could also be about changing the overall tone of the story. Instead of Issei's more comical and pervy attitude driving parts of the plot, the fanfiction might have a more serious or different comical approach by removing or reducing his presence.
He might react with hostility. Dark Naruto has a certain edge to him, and if he sees Rias as a threat or someone he doesn't like, he'll likely oppose her. He could use his combat skills to challenge her directly.