Power dynamics play a big role too. Naruto is a very powerful ninja, and in the High School DxD universe which is filled with powerful devils, angels, and other supernatural beings, how his power compares and how he uses it is a major aspect. He might have to learn new ways to use his chakra in a world where magic and sacred gears exist.
A good Naruto and High School DxD fanfic would first have well - written characters. The author needs to understand the personalities of the characters from both series and make them act in a way that is true to their original selves. Also, an interesting plot is crucial. It could be something like a common threat that forces the characters from both universes to work together. For example, a new powerful enemy that neither Naruto's ninja skills nor High School DxD's magic can easily defeat.
He could be a new type of threat or ally in the DxD world. Since Naruto has such powerful abilities, the fallen angels might see him as a potential weapon or a hindrance to their plans.
One idea could be to create a fanfic where Naruto is somehow transported to the world of High School DxD. He could bring his unique ninja skills and chakra system. Maybe he teams up with Issei to fight against a common enemy. For example, a powerful devil or fallen angel that threatens both the ninja world and the DxD universe.
Character development is a key element. For example, seeing how Kalawarner changes and grows in the high school environment. Another element is the integration of the DxD universe's supernatural aspects into the high school setting. Like how magic or demonic powers play out in the hallways.
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.
You could write a fanfic where the two worlds collide due to a powerful magic or jutsu gone wrong. The characters from both Naruto and High School DxD have to team up to fix the problem. This would bring together different personalities and abilities. For instance, Issei's perverted nature could be a source of comedic relief while Naruto's determination could inspire the DxD characters to fight harder.
In this fanfic, Naruto's unique abilities like his shadow clones and Rasengan could be combined with the magic and powers in the DxD universe. For example, a fallen angel might be interested in Naruto's chakra and try to use it for their own purposes. Naruto, being the hero he is, would not let that happen and would start a conflict that could lead to an epic adventure.
A fanfic of this nature could explore different themes. One theme could be the clash between different power systems, that is, the ninjutsu from Naruto and the magic in High School DxD. The primordial god connection could act as a bridge or a source of conflict. Another theme could be about Naruto's journey of self - discovery in this new and strange world. He might have to adapt to the different social and power structures in High School DxD while also coming to terms with his new role as someone related to a primordial god.
One possible fanfic could be about Naruto being transferred to the High School DxD universe and his encounter with the Juubi there. Maybe he has to team up with the characters from High School DxD to deal with the threat of the Juubi. For example, Naruto's unique abilities like the Rasengan and Shadow Clone Jutsu could be a great asset in battles within this new universe.
One great 'Naruto - High School DxD' fanfic is 'Naruto in the World of High School DxD'. In this fanfic, Naruto is somehow transported to the world of High School DxD. It's interesting to see how his ninja skills and values from the Naruto world interact with the supernatural and often chaotic world of High School DxD. The author does a good job of integrating Naruto's character traits into the new setting, creating a unique and engaging story.