A third plot idea could be a time - travel plot. A character from the 'God of Highschool' accidentally travels back in time and has to navigate the past while also trying not to change the future too much. They could meet younger versions of the other characters and have to deal with different relationships and power levels.
One idea could be exploring a new Godly power that the main character discovers within the Highschool DxD universe. Maybe it's a power that has been long forgotten and its emergence shakes up the existing power dynamics among the devils, angels, and fallen angels.
One idea could be to have the characters from High School DxD cross over into a world ruled by different gods. They could be tasked with a quest to restore balance. For example, Issei could use his Boosted Gear in a realm where the gods' powers are waning and his power might be the key to saving the day.
A plot could revolve around an OC Dragon God who forms an unlikely alliance with Issei. They could team up to face a greater threat that neither the devils nor the angels are fully aware of. The OC could bring in their unique dragon god powers and knowledge to help Issei level up in a different way, adding a new dynamic to the story.
Well, an interesting plot might involve an OC Dragon God who has lost their memories and has to rediscover their powers and identity while interacting with the characters in Highschool DxD. They could start off as a seemingly ordinary student, but as the story progresses, strange things start to happen that hint at their true nature. Also, an OC Dragon God could come from a different dimension and bring with them unique powers and knowledge that change the dynamics of the Highschool DxD world. Maybe they have a special connection to one of the main characters that has been fated since long ago.
One idea could be a dragon disguised as a new student in high school. It tries to fit in while hiding its true identity and dragon powers. Another could be a group of students who discover a secret dragon lair beneath their high school and have to protect it from those who would misuse the dragon's power.
One idea could be a story where Naruto and Sasuke are rivals in high school sports. They compete in track and field events, with Naruto's determination against Sasuke's natural talent. Another could be about Hinata being the shy new girl in school and Naruto slowly befriending her and helping her come out of her shell. Maybe there's a high school ninja club where they all learn and practice ninjutsu in a modern setting.
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 highschool talent show where Sasuke is forced to participate by the school and Sakura helps him overcome his shyness about performing. Another could be a mystery in the highschool where Sasuke and Sakura team up to solve it, like a series of stolen textbooks. Or perhaps a sports competition where Sasuke is on the track team and Sakura is the team's medic, and they develop a deeper bond through the event.
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.