Perhaps Shinji Ikari could be reimagined as a devil in the Highschool DxD world. He would have to adapt to the devil society, with its hierarchies and power struggles. His introverted nature might be a contrast to the more flamboyant devils, and his journey to find his place within this new world could be a central part of the fanfiction.
One idea could be that Shinji Ikari is transferred to the Highschool DxD school as an exchange student. He might initially struggle with the very different supernatural environment compared to his Evangelion world. His experiences with Angels could give him a unique perspective on the devils and other beings in the Highschool DxD world, leading to some interesting interactions and storylines.
He could also become more social. In the Highschool DxD world, there are many different species and characters to interact with. Shinji, who is often isolated in Evangelion, might start to open up and form friendships or alliances. This would be a big change from his usually withdrawn personality.
One idea could be a crossover with a superhero universe. Shinji Ikari could gain superpowers and have to navigate the complex world of superheroes, dealing with their moral codes. Another could be a crossover with a fantasy world like Middle - earth. He might find himself on a quest with hobbits or elves. A third idea is a crossover with a post - apocalyptic world, where Shinji's experiences in his own apocalyptic - like setting of Neon Genesis Evangelion could be put to use in a different kind of desolate landscape.
One idea could be that Naruto is somehow transported to the world of Highschool DxD. He could use his ninja skills and chakra in the new world, maybe even teach some of the characters there a thing or two about combat. For example, he could show Issei how to better control his power using chakra control techniques.
These fanfictions expand the Highschool DxD universe by introducing new characters like male Nekoshou. For example, they can create new storylines around their powers and how they interact with the existing characters in the DxD world. It might be a male Nekoshou with a special ability that changes the power dynamics in the school.
Yes, there could be such fanfictions out there. Fanfiction writers often like to cross over different popular franchises, and combining Naruto and Highschool DxD in this way is not unheard of. It allows for some really interesting character interactions and plot developments.
There are fanfictions where Shinji Ikari has different relationships with the other characters. In some, he forms a closer bond with Asuka earlier on and they work together more effectively against the Angels. This changes the whole dynamic of the story as their combined strength and new - found friendship alters the course of events.
One idea could be that the characters from Highschool DxD end up in the Hellsing universe. Maybe Issei and his friends are mistaken for new types of vampires due to their devil powers. They then have to team up with the Hellsing organization to clear their names and also fight against a common enemy that is threatening both the devil and human worlds in this combined universe.
A more complex fanfiction could be about a joint mission between the two universes. The leaders of both worlds realize that there is a common threat that only the combined forces of Highschool DxD and Naruto can defeat. So, they send their best characters, like Rias and Sasuke, to work together. This would involve a lot of character development as they learn about each other's cultures and powers.
A story could also revolve around a quest that involves both Diablo and Highschool DxD elements. The characters from both universes could team up to find a powerful artifact that has significance in both worlds. They would need to overcome their differences and work together, with the devils and angels from Highschool DxD learning about the dark and dangerous forces from Diablo.
One idea could be a crossover where Naruto characters are transported to the world of The Highschool of DxD. For example, Naruto himself could end up at the high school and his ninja skills would be quite a shock to the devils and other beings there. He might get involved in the Rating Games, using his shadow clones and other jutsu to his advantage.