It could be due to a curse or a blessing. Maybe in the process of traveling between the Naruto universe and the DxD universe, some sort of magical energy latched onto Naruto and made him immortal. For example, it could be related to the Sacred Gears in the DxD world. A Sacred Gear could have malfunctioned or had an unexpected reaction when Naruto entered the world, granting him immortality.
His immortality might stem from his own chakra. In the Naruto universe, chakra is a powerful energy source. When he arrives in the DxD world, his chakra could interact with the ambient magic in that world in a way that halts his aging process and makes him immune to fatal injuries. This could be because his chakra has a unique frequency that the DxD world's magic recognizes as something that should be preserved. Naruto could then have to learn to control this new aspect of his existence while dealing with the various challenges in the DxD world.
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.
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.
Naruto's reaction could be a mix of sadness and resolve. He has a big heart, so the betrayal would cut deep. But he also has the will of fire within him. He could use this as fuel to rise above the situation. He might seek out new allies in the High School DxD world, like Issei and his friends (if they weren't part of the betrayal). Together, they could work towards uncovering the truth behind the betrayal and taking on any threats that might be related to it.
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.
A fanfiction might explore how the power of the Excalibur in High School DxD would interact with Naruto's chakra. Since the Excalibur is a powerful holy weapon, it could have some interesting reactions when in the presence of Naruto's unique chakra, which is different from the magic in the High School DxD world. For example, it could either enhance or nullify certain abilities depending on how the author writes it.
There could be. Fanfiction writers are known for their wild and creative ideas. Since both Naruto and High School DxD are popular series, someone may have written a fanfic where Naruto is presented as a god - like figure within the High School DxD universe. This could add an interesting twist to the story, perhaps bringing in Naruto's unique powers and abilities into the High School DxD world.
In a Naruto - High School DxD fanfiction, the power levels would be really interesting to explore. Naruto's sage mode and tailed - beast forms are extremely powerful. In the DxD world, there are different power systems like the Sacred Gears. It would be fascinating to see how Naruto's powers interact with the powers of the DxD characters. For example, could his Rasengan be enhanced by a Sacred Gear? Or how would he fare against a devil's magic?
One possible scenario could be that Naruto characters are somehow transported to the world of High School DxD. Naruto's ninjutsu skills would be quite a shock to the devils and angels in that world. For example, Sasuke's Sharingan could give him an edge in battles against the supernatural beings in High School DxD. And Naruto's never - give - up attitude might inspire some of the characters there to face their challenges more bravely.