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.
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 idea could be a story where the viser has to use their unique powers to protect the main character in a dangerous situation within the Highschool DxD universe. This could lead to a deepening of their relationship and some emotional moments.
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.
A really interesting plot could be Gasper getting transported to a different dimension or a parallel world within the DxD universe. In this new place, he has to rely on his wits and the basic powers he already has to survive and find a way back home. Along the way, he meets different versions of the characters he knows, and through his interactions, he learns more about himself and his place in the grand scheme of the DxD world.
To write good character development for Murayama, start by establishing her basic traits. For example, if she's initially shy in the fanfiction, show how she gradually becomes more confident as she faces challenges in the Highschool DxD world. Maybe she has to speak up in front of devils or other powerful beings, which forces her out of her shell.
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 plot idea could be Natsu accidentally ending up in the Highschool DxD world and having to adapt to the different rules and magic systems there. He could make friends with some of the main characters and they could team up to fight a common enemy that threatens both his world and the Highschool DxD world. Another idea is that Natsu's unique brand of magic could be a mystery to the characters in Highschool DxD and they try to figure out how it works while also teaching him about their own powers.
A more complex plot could be centered around a prophecy that foretells the destruction of the Highschool DxD world. The characters then have to embark on a journey to find the source of this prophecy and try to prevent it. Along the way, they discover secrets about their own powers and the true nature of the different factions, and have to make difficult choices that could either save or doom their world.
A third plot idea might be Sona trying to modernize the traditions of the devil world within the context of the school. She could face opposition from more conservative devils, and the story could explore her struggle to bring about change while still maintaining her position and respect.
One idea could be a new character who is a killer but has a hidden noble purpose. Maybe they are targeting rogue devils that are harming humans secretly. This would add an element of mystery and action to the fanfiction.