One interesting plot idea could be a new character born from the fire element who has unique powers that disrupt the balance in the Highschool DxD world. For example, they might be able to control a type of fire that is immune to the usual magic defenses. Another idea could be a story where the main character discovers a hidden history related to the 'born of fire' concept in the Highschool DxD universe, like an ancient prophecy or a long - lost clan.
Well, perhaps a plot where the 'born of fire' character is actually an experiment gone wrong by some secret organization in the Highschool DxD world. This character then has to figure out their origin while also dealing with the various factions in the school, like the devils, angels, and fallen angels. Or it could be a story about a 'born of fire' entity that is reincarnated and has memories of a past life that are crucial to saving the Highschool DxD world from a new threat.
One idea could be a story where Issei and Rias have to face a new, powerful enemy that targets their relationship. Maybe an ancient demon who sees their bond as a threat to the traditional demon hierarchy. Another plot could be a time - travel story where they go back in time and have to deal with past events that could change their present relationship. For example, they might have to stop an event that would have otherwise led to their separation.
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.