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.
A third plot idea is that Rias's little brother has a dream of uniting the different devil clans. He believes that the constant infighting is weakening them all. So, he embarks on a journey to meet with the leaders of the other clans. Along the way, he faces many challenges and prejudices because of his young age and his relation to Rias. But he doesn't give up and tries to convince them through his ideas and his own growing strength.
Well, a fanfiction could focus on their daily lives outside of the battles. For example, Rias and Issei could have a cooking competition with their friends from the Occult Research Club. It would be a fun and light - hearted way to show their relationship and how they interact with the other characters in a non - combat setting.
One idea could be a story where Issei and Rias have to face a new, powerful enemy that targets their relationship. Maybe an ancient demon who sees their bond as a threat to the traditional demon hierarchy. Another plot could be a time - travel story where they go back in time and have to deal with past events that could change their present relationship. For example, they might have to stop an event that would have otherwise led to their separation.
One idea could be that Issei leaves Rias because he realizes he has different goals in the supernatural world. He might want to focus on helping the weaker devils rather than being part of Rias's peerage. So he sets out on his own journey, making new allies and facing new challenges.
One idea could be that the OC has a unique power that helps Rias and her harem in unexpected ways. For example, the OC might be able to sense danger before it arrives, allowing them to better prepare for battles against other factions. Another plot could involve the OC having a past connection to one of the members of Rias' harem, which creates some drama and tension as they navigate their new relationships within the harem. Maybe the OC is a long - lost relative or an old friend who has changed a great deal.
Well, a possible plot could be Rias having to balance her pregnancy with her studies at school. She's not just a devil but also a student. There could be comical moments like when she has sudden cravings during class or struggles to fit into her school uniform as her body changes. Also, the relationship between Rias and Issei could deepen in new ways. They might have to face challenges together, like dealing with over - protective family members from both the human and devil sides.
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 fanfiction could explore their past. For example, how they first met in the underworld. It could show their initial impressions of each other and how their relationship developed over time. Perhaps there was some sort of competition between them at first, but they later realized they could be great friends and allies. This would give readers a deeper look into their characters.
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 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.
One idea could be that the male OC has a unique power that Rias needs to protect her peerage. So they start working closely together, and slowly feelings develop. For example, the OC might be able to sense impending danger to the group in a way no one else can.