Another approach is to mix the power systems. In fanfiction, the chakra from Naruto could be related to the power of the devils or other beings in High School DxD. For instance, the tailed - beast chakra could be equivalent to a powerful devil's power source. Characters from both universes could then try to learn about and utilize the other's power system. This would create a lot of interesting scenarios where they have to train together or compete to see who has the stronger power.
A third way is through crossovers of storylines. The overarching plot of Naruto, like the fight against the Akatsuki or the goal of achieving peace in the ninja world, could be intertwined with the story of High School DxD, such as the battles against the fallen angels and the struggle for power among the different factions. For example, the Akatsuki could be working with some of the enemies in High School DxD to achieve their own goals, and Naruto and his friends would have to team up with the High School DxD protagonists to stop them.
One way to combine them could be by having Naruto characters enter the world of High School DxD. For example, Naruto with his determination and unique abilities could interact with the devils and other supernatural beings in the DxD world. Maybe he'd form an unlikely alliance or even compete in the Rating Games.
One way is through the concept of power transfer. For instance, a character from Naruto could accidentally gain the power of a devil in High School DxD and have to figure out how to control it. This would lead to some really cool action scenes as they struggle with the new power. Another way is through cultural exchange. The shinobi culture from Naruto could be introduced to the High School DxD characters, creating some interesting interactions.
They can be combined by having a shared plot device. Maybe a powerful artifact that is important in both Naruto's world and the High School DxD world. This could bring the characters from both series together. The ninjas could be interested in the artifact for its power to protect their village, while the devils and angels in High School DxD might want it for their own purposes like increasing their power or influence. It could also lead to some characters from each series having to team up to protect or obtain the artifact.
In a Naruto and High School DxD fanfiction, there could be a scenario where the protagonists of both stories meet in a neutral dimension. Issei from High School DxD might be intrigued by Naruto's ninja skills and determination. Naruto, in turn, could be curious about Issei's Sacred Gear. They could start training together, sharing their knowledge and skills, and ultimately have to face a common enemy that threatens both their worlds.
Also, in terms of character relationships, a fanfiction might pair characters from both series in unexpected ways. Maybe a character from Naruto forms an alliance with a devil in High School DxD against a common enemy. This could lead to interesting dynamics and development of new storylines.
It can work by finding common elements between the two. Both have elements of power systems, like chakra in Naruto and Sacred Gears in High School DxD. So, a story could revolve around comparing or combining these power systems.
One way could be through the power system. The EMS (Eternal Mangekyou Sharingan) from Naruto could be a unique power that a character in the High School DxD world acquires. Maybe a transfer student from a hidden ninja village arrives at the high school and has to keep their EMS a secret while dealing with the devils and angels in the school. This could lead to some interesting battles and interactions as the powers of the EMS, like the ability to use powerful jutsus, contrast with the magic and abilities of the High School DxD characters.
One possible fanfiction could be about Naruto characters attending the school in High School DxD. For example, Naruto's positive and determined attitude might clash with the more complex social and power dynamics in the DxD world. He could make new friends or even rivals among the devils, angels, and fallen angels.
One possible fanfiction could be about Naruto characters somehow ending up in the world of High School DxD. Maybe Naruto's skills in combat and his determination would make him a formidable opponent in the battles that often occur in the High School DxD universe. He could form interesting alliances or rivalries with the characters there.
A fanfiction could focus on Naruto's training regime influencing the characters in High School DxD. His intense physical and mental training could inspire the students at the school. They might start incorporating some of his training methods, like running up trees or on water, to improve their own strength. And Naruto could also learn from their magic-based training and combine it with his chakra control for new and powerful techniques.