Some fanfictions might focus on the emotional aspect of the One Winged Angel. How does the character who has this form deal with the power, the isolation it might bring, or the new relationships that form as a result of it? This can add a lot of depth to the story.
Rias Gremory could also be a main character. Her relationship with Issei and her role as a devil leader means she can play a crucial part in fanfictions about the One Winged Angel. She might be involved in helping Issei master the form or dealing with the consequences of its appearance.
In Gilgamesh Highschool DxD fanfiction, there might be concepts like Gilgamesh forming alliances or rivalries with the devils, angels, and fallen angels in the Highschool DxD world. His vast treasures from his own story could also play a role, perhaps being used as bargaining chips or powerful weapons in the battles that often occur in the Highschool DxD setting.
One interesting concept could be the crossover of the Highschool DxD world with the eerie and mysterious Slender Man. For example, how the characters in Highschool DxD, with their unique powers and personalities, would react to the presence of the Slender Man. Maybe the devils in Highschool DxD would try to make a deal with the Slender Man or fight against him to protect their territory.
In Aizen Highschool DxD fanfiction, a cool concept might be how Aizen's Zanpakuto abilities would translate in the Highschool DxD setting. His Shikai and Bankai could have new and unexpected effects. Maybe his illusions could be used to deceive not just in battle, but also in the social and political intrigue that is so common in the Highschool DxD world among the different factions.
In some Highschool DxD SI Fanfiction, the self - insert might come from a different world or time period. This brings in elements of cultural shock and different knowledge systems. They could introduce new technologies or fighting styles to the world of Highschool DxD, which can lead to very creative and engaging storylines.
One interesting concept could be a crossover where the characters from Highschool DxD encounter Slenderman. Maybe the devils in Highschool DxD are intrigued by Slenderman's otherworldly and spooky nature. They might try to either recruit him or fight him, depending on their motives.
One interesting concept could be the exploration of different character relationships in a more subdued or 'silent' way. For example, how the main character might form unexpected friendships without the typical high - energy drama. Another could be the re - imagining of the story's settings in a more peaceful or reflective manner.
One interesting concept could be the combination of Pokemon powers with the abilities of the characters in Highschool DxD. For example, a character might be able to use a Pikachu's electricity - based attacks in combat situations within the Highschool DxD universe. Another concept could be the idea of Pokemon existing in the same world as the devils, angels, and fallen angels in Highschool DxD. This could lead to some really unique interactions, like a Pokemon being tamed by a devil and used as a sort of familiar or ally in battles against other factions.
Another concept could be the exploration of different power systems. Hellsing has its own set of rules regarding vampire powers, holy weapons, etc., while Highschool DxD has the Rating Game system and the various devil powers. Fanfiction might play around with how these power systems interact, like if a character from Hellsing were to participate in a Rating Game or how a devil from Highschool DxD would fare against a Hellsing vampire.
Another concept is the exploration of different relationships. In highschool dxd system fanfiction, characters might form new alliances or romances that weren't as prominent in the official story. This can lead to some really engaging character development.