Another idea for the fanfic could be a prequel of sorts. It could show their childhood and how their bond was formed. Maybe there were early challenges they had to face together, like dealing with the expectations of their noble devil families. This could also delve into their training as young devils and how they influenced each other's growth and personalities.
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 fanfic could focus on a particular adventure where Sirzechs and Rias team up. They might have to solve a mystery in the Underworld that has the potential to disrupt the balance between the three factions (devils, angels, and fallen angels). This would give the author a chance to show their individual powers and how they work together. Rias' intelligence and Sirzechs' strength could be combined to overcome various 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 great fanfic could be 'Sirzechs' New Adventure'. It delves into her character in a unique way, exploring her relationships and powers. Another is 'Female Sirzechs: A Different Path' which shows a different side of her compared to the original series.
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.
One possible fanfiction could be about Rias and Sona teaming up for a school event. They might put aside their usual competitiveness and work together to organize a devil - related festival at the school. They could combine their unique skills and powers to create an amazing event that not only impresses the other students but also strengthens their bond as friends.
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.
I haven't read this particular fanfiction yet, but generally, a fanfiction about the son of Sirzechs in the Highschool DxD universe could be really interesting. It might explore new powers, relationships, and storylines that the original series didn't cover. For example, how the OC would interact with the existing characters like Issei and Rias, and how his heritage as the son of Sirzechs would shape his destiny in the Devil society.
For Rias' birthday, the entire school seemed to be in a festive mood. The ORC members had a special plan. They arranged a tournament in honor of Rias. Devils from different factions came to participate. Rias watched with pride as her friends fought bravely. In the end, Issei emerged victorious. As a birthday present, he gave her a necklace with a small ruby, symbolizing their bond. Rias was deeply touched and put on the necklace immediately, her eyes shining with happiness.
In Sirzechs and Rias fanfiction, the relationship between the two can be developed in various ways. It could be a more brother - sisterly bond, or a relationship based on mutual respect for their powers. Writers can play around with their abilities and how they use them in different situations. This can lead to really engaging stories that expand on the world of the original work.