A cool fanfiction could also focus on a love story between a character from Highschool DxD and an exorcist. How they deal with their different backgrounds, duties, and the potential disapproval from their respective groups would make for an engaging plot.
One idea could be an OC who is an exorcist with a special power to see the true forms of devils. They could start off as an enemy to the devils in the Highschool DxD world but then gradually form alliances due to a common threat. For example, a new, more powerful and evil force emerges that targets both exorcists and devils.
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.
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.
A third idea could be a time - travel fanfiction. For example, a character from Highschool DxD travels back in time to the era of Bleach and meets Ichigo when he was still developing his powers. This could lead to some interesting interactions and changes in the timeline of both universes.
Another interesting concept could be a character swap. Imagine Ichigo from Bleach attending the school in Highschool DxD. How would he interact with the devils and other supernatural beings there? He might use his Soul Reaper powers in unexpected ways within the Highschool DxD universe.
You could write about an OC who is a half - angel, half - devil. Their internal struggle between the two natures and how they fit into the world of Highschool Dxd, trying to find their place among the devils, angels and fallen angels. It could explore themes of identity and acceptance.
Another interesting concept is to set the fanfiction in an alternate universe. In this universe, the twins could be from a different race or have a different upbringing. This would allow for a fresh take on their characters and how they fit into the Highschool DxD world. For example, they could be raised in a human - devil hybrid society, which would give them a different perspective on the ongoing conflicts in the series.
One idea could be creating a new character who is a transfer student with a unique power. For example, a character who can manipulate time in the Highschool DxD universe. This could lead to all sorts of interesting plot twists, like changing the outcome of battles or getting a peek into the past of the main characters.
You could write a fanfiction where the Akatsuki are summoned to the Highschool DxD world by a powerful magic user. They then get involved in the power struggles between the different factions. Maybe they side with the devils at first, but then realize that the angels also have some good points. The interactions between the Akatsuki members and the characters from Highschool DxD would be really interesting to explore, like how Sasori might react to the technology in the Highschool DxD world compared to his puppet - making skills.
One idea could be a scenario where the main characters have to fight against a horde of zombies that are created by an evil sorcerer who wants to take over the school. The zombies could have unique abilities based on the different races in the Highschool DxD universe.