A plot could involve a hidden tournament within the school that only a select few know about. The main character accidentally discovers it and has to train hard to compete. Or perhaps a story where a character loses their powers and has to find a way to regain them while dealing with the challenges of high school life in this universe. It could also be about a mysterious object that appears on campus and causes all kinds of chaos as different factions try to obtain it.
One idea could be a new character arrives at the school who has a unique power that disrupts the existing power balance among the student groups. Another could be a time - travel plot where a character goes back in time to change an important event in the Highschool DxD universe. Maybe a crossover with another popular anime, where characters from both shows interact in the Naruko Highschool DxD setting.
Well, you could write a fanfiction about Naruko's arrival in the DxD world and how she adjusts to the different culture and rules there. It could be really fun to see her reactions to things like the Rating Games or the different devil clans. Or perhaps a story where Naruko tries to teach some of her ninja techniques to the DxD characters, and they in turn teach her some of their magic.
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.
Maybe Jesus could have a special mission in the Highschool DxD universe. For example, he might be there to stop a great evil that is brewing and uses his powers in a way that is different from the traditional powers in the series. He could team up with Issei and the others, bringing a new dynamic to their group.
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.
A possible plot is that a group of students in High School DxD start a gaming club, but they accidentally discover that the games they play are linked to some ancient and powerful magic. As they delve deeper into these games, they uncover secrets about their world and have to face off against enemies who want to use this magic for evil purposes.
One idea could be a new character who is a futa and has unique powers related to both male and female aspects. They could join the Gremory group and cause some interesting dynamics, like having a different approach to battles and relationships within the group.
One idea could be exploring Kuroka's past more deeply in the High School DxD universe. Maybe she has some hidden relationships or secrets from her time before joining the main story's events.
Another interesting plot could be about a joint mission between the two universes. The characters from both Nasuverse and Highschool DxD need to work together to defeat a common enemy that threatens both of their worlds. This would require them to understand each other's abilities and form strategies that combine their unique powers.