One interesting plot idea could be Murayama discovers a hidden power within herself. Maybe she has a connection to an ancient artifact in the Highschool DxD universe that gives her unique abilities, like the power to control time briefly. This could lead to her having to protect this secret from both the devils and the angels in the story.
One interesting plot idea could be Murayama getting involved in a magical mystery within the Highschool DxD universe. Maybe she discovers a hidden power and has to learn to control it while dealing with the normal high school drama and the supernatural elements around her.
The combination of their characters. Issei is a complex character with his own set of powers and desires in the Highschool DxD universe. Murayama, on the other hand, has her own charm. When they are paired in fanfiction, it can lead to unique storylines that explore different aspects of their relationship, whether it's friendship, a potential romantic relationship, or something else.
One great fanfiction could be where Murayama discovers Issei's true nature as a devil early on. This could lead to some really interesting adventures as she tries to come to terms with it. Maybe she starts to train with him to be able to protect herself in the dangerous world of devils and fallen angels.
One way to develop her character is to have her learn new skills. For instance, she could start training in martial arts or some form of magic that is accessible to humans in the Highschool DxD world. As she learns, she could face setbacks and challenges that force her to grow both as a fighter and as a person.
A plot could also focus on a new character who is a hybrid of the two universes. They have the powers of both a devil from High School DxD and a wizard from Harry Potter. They struggle to find their place in either world as they are too different from the norm in both, and they embark on a journey to create their own identity and purpose.
One idea could be exploring new power-ups for the characters. For example, Issei could discover a hidden ability within his Boosted Gear that allows him to time - travel briefly during battles, adding a new strategic element. Another could be focusing on the relationships in the future. Maybe Rias and Issei have a family and the story could revolve around their children's upbringing in the supernatural world.
One idea could be a new character joining the school who has a unique supernatural ability that no one has seen before. This could disrupt the existing power dynamics and create all sorts of interesting interactions and challenges.
One idea could be to have a new character enter the world of Highschool DxD who challenges the existing power hierarchy and calls out the flaws in the main characters' actions. For example, this new character might see Issei's pervy ways as not just a comical trait but something that is disrespectful to the female characters and start a movement to change the way things are done in the school.
Well, perhaps a fanfiction where Griselda gets transported to a different dimension. In this new place, she has to adapt quickly and find a way back home. Along the way, she meets new allies and enemies. This would be a great way to explore her character in a different setting.
One idea could be a succubus character trying to resist her natural instincts in the high school setting of Highschool DxD. For example, she might struggle with not using her powers to manipulate the male characters around her.