A plot idea could involve a character from Highschool DxD getting lost in the world of Trinity Seven and having to adapt to their magic system. They could learn new spells and make new friends along the way, all while trying to find a way back home. It would be a great opportunity for character development and exploring the differences between the two universes.
A plot could revolve around a time - travel fanfiction. Maybe a character goes back in time and has to prevent an event that would have a catastrophic effect on the future of the Trinity Seven world. This could involve interacting with younger versions of the characters and changing their fates.
Perhaps a crossover where the Trinity Seven characters enter a post - apocalyptic world. Their magic would be both a great asset and a source of danger as other survivors might see them as threats or saviors. They'd have to figure out how to survive and maybe even find a way to restore the world using their powers.
Another interesting concept is an OC who is from a completely different dimension but gets drawn into the Trinity Seven world. This OC could have a different set of values and magic rules from their own dimension, which would create a lot of interesting interactions and conflicts with the existing characters.
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.