One common theme is identity. Since the twin is often in the shadow of Rias, exploring their own identity becomes important. Another theme is power. Many fanfictions focus on the unique powers the twin may possess.
One popular 'Highschool DxD Rias Twin' fanfiction is 'The Twin's Bond'. It explores the relationship between Rias and her twin in a unique way, adding new storylines to the Highschool DxD universe.
Well, if Naruto were Rias' twin brother, it would completely change both storylines. In Naruto's world, his new family connection to the Gremory family in High School DxD might introduce devilish powers and politics into the Hidden Leaf Village. This could lead to new alliances and rivalries. For example, the Akatsuki might view Naruto differently, seeing him as a potential link to a powerful devil family. In High School DxD, the devils might be more involved in the affairs of the ninja world, perhaps trying to gain access to unique ninja techniques. It would also change Naruto's and Rias' characters. Naruto might grow up with a different sense of self, having a devil heritage, and Rias would have a strong connection to the ninja world through her brother.
A great fanfic idea could be that Rias and Riser are sent on a joint mission to the human world. They have to blend in as high school students. This would be a comical and interesting premise as they struggle to adapt to human life while also dealing with their own relationship. Rias might be more empathetic towards humans, while Riser could be more arrogant at first.
One possible fanfic could be about a friendly rivalry between Rias and Riser that turns into a mutual respect. For example, they could start competing in various magical challenges within the Highschool DxD universe. This would show their growth as characters and how they overcome their initial differences.
One great fanfiction idea could be that Issei discovers a new power within himself that is directly linked to Rias's heritage. Maybe he can tap into an ancient magic that only works in harmony with Rias's own powers. As they train together to master this new ability, their relationship deepens beyond just the typical master - servant bond in the Highschool DxD world.
In the case of 'dxd rias bashing fanfiction', it could be centered around different aspects. It might be that the author has a different vision for the 'dxd' world and Rias doesn't fit into that vision as she is in the original. So they create a fanfiction to bash her character. It could also be due to personal preference of the author. They might prefer other characters over Rias and use the fanfiction as a platform to show why they think Rias has negative qualities. Another possibility is that they want to explore how the story would change if Rias was not the 'perfect' character as she seems in the original, so they write a bashing fanfiction to break her down and see what new stories can emerge.
A common theme is power - up. Rias may gain new powers or improve her existing ones. This could be related to her devil heritage or through some special training or encounter.
In some cases, it could be that the author of the fanfiction simply wants to explore a different narrative where Rias is not the 'perfect' character she is often portrayed as in the main series. They might want to create a more complex or alternative view of the Naruto DxD universe by highlighting what they see as flaws in Rias's character.