One possible plot could be a cultivator arriving in the DxD world and being mistaken for a powerful devil. They might have to navigate the complex politics of the three factions while also trying to find a way back home. Along the way, they could introduce some of their cultivation techniques to the local characters, which would cause a stir as they are so different from the magic and power systems already in place.
A female cultivator could be the main character. She might be on a quest for revenge in her own world but gets sucked into the DxD world instead. In the DxD world, she has to start from scratch in a sense. She uses her wits and cultivation skills to survive and eventually rise to power. Her gender could also play an interesting role as she might face different challenges compared to a male character in a world that also has its own ideas about gender roles and power dynamics.
One possible plot could be Naruto being summoned to the DxD world by some sort of magical accident. He could then use his ninja skills, like his shadow clone jutsu, in unique ways to combat the devils and exorcise evil spirits. For example, his clones could be used to surround and confuse enemies during an exorcism battle.
Well, a plot could involve Broly being part of a special program in the Highschool DxD setting. He might be there to learn about controlling his power in a more civilized way. As he tries to fit in, he could befriend some of the main characters from DxD. For example, he could team up with Issei and his friends to compete in some sort of school tournament that has magical and combat elements. This would lead to some epic battles and also some character development for Broly as he learns about friendship and cooperation.
Maybe Spider - Man's identity is mistaken in the Highschool DxD world. The characters might think he is some sort of new supernatural being. This could lead to a series of comical misunderstandings as he tries to explain his powers and where he comes from while also dealing with the various power struggles and intrigues in the Highschool DxD universe.
One possible plot could be Percy arriving in the DxD world and being mistaken for a new, extremely powerful devil. He then has to navigate the complex politics of the devils while also trying to find a way back home. Along the way, he uses his demigod powers in unique ways against the various threats in the DxD universe.
It could be about the characters having to create their own moral code in a world without a divine being to guide them. Maybe the devils and angels would have to re - evaluate their long - standing enmity and find new ways to coexist.
One idea could be that Superman, with his powers, becomes a sort of protector in the High School DxD world. He could use his super strength to stop any over - the - top fights that break out between the different factions. And his heat vision might come in handy when dealing with some of the more dangerous magical threats that lurk in the shadows of the school.
One idea could be a story where Issei and Rias have to go on a special mission together in a parallel dimension. In this dimension, they lose their powers and have to rely on their wits and the bond they've developed. They face challenges that test their relationship and force them to understand each other on a deeper level.
Maybe in this fanfiction, the Nine - Tails in Naruto is actually a super - powerful Sacred Gear that was sealed within Naruto. This Sacred Gear has its own consciousness like in DxD. The story could revolve around Naruto trying to master this new form of power that is a blend of his chakra and the Sacred Gear's abilities. He would have to deal with the various factions in the DxD world who are interested in his power.
One possible interesting plot could be Sasuke arriving in the Highschool DxD world during a time of chaos. Maybe there's a new threat that neither the devils nor the other factions can handle easily. Sasuke's ninja skills and Sharingan would give him an edge in combat. He could form an alliance with Issei and his group, teaching them some ninja techniques in return for learning about the supernatural powers in their world.