Well, you could have a story where Mordred has a past connection to some of the powerful families in the Highschool DxD universe. Maybe she's on a quest to uncover something about her heritage that's linked to that world. And during her journey, she falls in love with one of the characters in an unexpected way. For example, she starts off as rivals with Rias but then they develop a deeper bond as they face challenges together.
A great idea for a Mordred x Highschool DxD fanfiction would be a crossover of their different power systems. Mordred's abilities from her origin story could interact in strange and exciting ways with the Sacred Gears in Highschool DxD. For instance, what if Mordred's swordsmanship was enhanced by a Sacred Gear? This could lead to a whole new arc where she has to master this new combined power while also dealing with her growing feelings for a certain character in the Highschool DxD world.
To develop Mordred in a Mordred x Highschool DxD fanfiction, her backstory can be explored further. Maybe she has some unresolved issues from her original story that resurface in the Highschool DxD setting. For instance, her complex relationship with her father figure. As she tries to come to terms with these in the new world, she could change and grow. She might also start to question her own values and beliefs as she experiences the different cultures and power dynamics in Highschool DxD, which would be an interesting way to see her character evolve over the course of the fanfiction.
One idea could be a crossover where Natsu finds himself in the world of High School DxD and has to deal with the devils and angels there. He might use his fire magic in unique ways to combat the various threats in that universe.
One idea could be to focus on the characters' relationships outside of the main plot. For example, how the students interact during normal school days, like study groups or after - school clubs. Another idea is to create an alternate universe where the rules of the dxd world are slightly different in the high school setting, which could lead to new adventures and challenges for the characters.
Perhaps there could be a crossover of the two schools. The students from Beacon Academy and Kuoh Academy could have an exchange program. Yang might get into some friendly (or not so friendly) competitions with the devils in terms of strength, while Weiss could be fascinated by the more formal and complex social structure in the Highschool DxD world.
Maybe in the fanfiction, there's a magical event that links the world of Illya with the Highschool DxD universe. Illya's innocent nature could be a contrast to the more risqué and wild nature of some of the Highschool DxD characters. Her friendship with Issei could start off rocky but then develop into a strong bond as they train together to improve their respective powers.
One idea could be a character from Prototype having to adapt to the supernatural world of Highschool DxD. For example, Alex Mercer could end up at Kuoh Academy and have to deal with the devils and angels while also trying to figure out his own powers in this new context.
One idea could be an OC who is a transfer student with a unique power that no one in the Highschool DxD universe has seen before. This power could attract the attention of the main characters and lead to various adventures and alliances.
One idea could be exploring a new character's journey in the Highschool DxD human world. Maybe a transfer student who has a unique power or ability that shakes up the status quo among the devils, angels, and fallen angels. They could start off as an unknown and gradually form alliances and rivalries.
One idea could be a story where the characters from High School DxD are reimagined as toddlers, and their innocent interactions lead to comical and heartwarming situations. For example, Issei might be chasing after a butterfly instead of chasing girls in his usual way.
A third idea could be a fanfiction where Gaara becomes a devil himself. He could bring his ninja skills and his sand abilities into the devil society. This would change the power dynamics in the DxD highschool as he would be a very different type of devil compared to the existing ones, with his own moral code and way of doing things.