The plot could also be centered on a mystery related to their family. The brother of Rias discovers some secrets about their family's past that put them both in danger. He then has to work with Rias to uncover the truth and stop whoever is trying to use their family secrets against them. This could involve battles, political intrigue within the Devil world, and self - discovery for both of them.
One possible fanfiction idea could be that Rias' sick brother is a key factor in a power struggle within their supernatural world. Naruto, being an outsider with his own set of skills and values, could get involved. Maybe he forms an alliance with Rias to protect her brother from those who would use his illness to their advantage.
Another plot could revolve around him getting involved in a school-wide mystery. Since he's related to Rias, he has access to certain areas and people. He starts to notice strange happenings at school that seem to be related to an ancient devilish artifact. As he delves deeper into the mystery, he forms unexpected alliances with other students, both devil and non - devil, and has to use his wits to solve the mystery before it's too late.
His character can be developed by giving him challenges. For example, he could be put in situations where he has to choose between loyalty to his family and what he believes is right. This would force him to grow morally. Also, having him face opponents stronger than him in battles would make him train harder and develop his combat skills.
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.