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 idea could be a story where Rias and Riser are forced to team up against a common enemy that threatens both of their peerages. This would put them in a situation where they have to set aside their differences and learn to work together. As they do so, they gradually start to understand each other better and a newfound respect develops between them.
Their relationship could be developed uniquely by having them exchange roles. For instance, Rias could take on some of Riser's responsibilities in the Underworld for a while, and Riser could do the same in Rias' territory. This would give them a deeper understanding of each other's worlds and the pressures they face. They would then be able to support each other in new ways, leading to a stronger relationship.
There's a fanfic called 'Rias and Issei: A New Adventure'. In this one, they go on a journey together that tests their relationship. The author does a wonderful job of portraying their feelings for each other and the challenges they face as a couple in the world of Highschool DxD.
One possible fanfic could be about Riser and Kuroka forming an unlikely alliance. Maybe they start off as enemies, but due to some common goal, like protecting the underworld from a new threat, they have to work together. Their interactions would be filled with tension at first, as Riser is quite prideful and Kuroka is independent. But as time goes on, they start to understand and respect each other more.
In this fanfic, Riser and Kuroka might find themselves in a race against time. They could be competing with other factions in the Highschool DxD world to reach a certain place first. Maybe it's a place full of ancient magic that could give them an edge. Their different abilities would play a crucial role. Riser's fire - based powers and Kuroka's unique nekoshou skills would be used in creative ways to overcome obstacles.
The plot could involve a power - transfer situation. Sirzechs, being a very powerful devil, might decide to transfer some of his power to Rias for a special mission. This would put Rias in a new position of strength but also bring new responsibilities. She would have to learn to control this new power while also dealing with the jealousy or suspicion of other devils.
One possible fanfic could be about their relationship as siblings in the context of the Highschool DxD universe. Maybe it could explore how Sirzechs' actions and decisions affect Rias. For example, his leadership in the Underworld might have an impact on Rias' development as a devil and her role in the school's Occult Research Club. It could also show how they support each other during difficult times, like when facing threats from other devil factions or powerful enemies.
A popular fanfiction is 'Issei and Rias: United Front'. It portrays them as a powerful duo against their enemies. The author does an amazing job of writing their interactions, making it seem very natural. The story also has some great plot twists that keep readers engaged. You can find these fanfictions on various fanfiction websites. Just search for 'Highschool DxD Issei x Rias fanfiction' and you'll get a whole list to choose from.