Perhaps a brave saint discovers a long - lost prophecy that involves her and Issei. She initially thinks Issei is a threat as he is a devil, but as she delves deeper into the prophecy, she realizes they might be key to saving the world from a greater, hidden evil. This could lead to a series of adventures where they have to learn to trust each other and combine their powers. Their relationship could start off rocky but gradually develop into a strong bond that surprises everyone around them.
A plot could involve a brave saint who is sent on a mission to make peace between the devils and the angels. She has to deal with the strong personalities on both sides, like Issei and his group of devils, and the high - ranking angels. She could start to question the long - held beliefs of her side as she gets to know the devils better. For example, she might find that not all devils are as bad as she was taught. Along the way, she forms unexpected friendships and alliances that change the dynamics of the three factions in the Highschool DxD world.
A powerful and corrupted brave saint could be the main antagonist. This character may have been corrupted by the allure of power or some dark magic. She believes that the brave saints should be the supreme rulers and decides to start a war against both the devils and the angels. She uses her knowledge of the brave saint techniques and her own new - found dark powers to cause mayhem and tries to eliminate anyone who stands in her way.
A plot could also focus on a new character who is a hybrid of the two universes. They have the powers of both a devil from High School DxD and a wizard from Harry Potter. They struggle to find their place in either world as they are too different from the norm in both, and they embark on a journey to create their own identity and purpose.
One idea could be exploring new power-ups for the characters. For example, Issei could discover a hidden ability within his Boosted Gear that allows him to time - travel briefly during battles, adding a new strategic element. Another could be focusing on the relationships in the future. Maybe Rias and Issei have a family and the story could revolve around their children's upbringing in the supernatural world.
One idea could be a new character joining the school who has a unique supernatural ability that no one has seen before. This could disrupt the existing power dynamics and create all sorts of interesting interactions and challenges.
One idea could be to have a new character enter the world of Highschool DxD who challenges the existing power hierarchy and calls out the flaws in the main characters' actions. For example, this new character might see Issei's pervy ways as not just a comical trait but something that is disrespectful to the female characters and start a movement to change the way things are done in the school.
Well, perhaps a fanfiction where Griselda gets transported to a different dimension. In this new place, she has to adapt quickly and find a way back home. Along the way, she meets new allies and enemies. This would be a great way to explore her character in a different setting.
One idea could be a succubus character trying to resist her natural instincts in the high school setting of Highschool DxD. For example, she might struggle with not using her powers to manipulate the male characters around her.
One idea could be a new character joining the school who has a mysterious power that both the devils and angels are interested in. This new character could disrupt the existing power balance in the school.
One plot idea could be Tiamat adjusting to the modern world in the Highschool DxD universe. Maybe she has to deal with the different social norms and the unique magic systems there.
One interesting plot idea could be the main character reincarnating as a powerful but unknown devil. They could then use their modern knowledge to revolutionize devil society. For example, introducing new forms of magic training or social systems.