Another idea is to have the Highschool DxD characters gain some of the shinobi abilities. Imagine Issei learning how to use chakra and performing basic ninja techniques. This could add a new dimension to his character and his battles, and also create some really interesting storylines as he combines his boosted gear with ninja skills.
Perhaps a crossover where the shinobi world and the Highschool DxD world are somehow connected. Maybe there's a portal that leads from Konoha to the school. This would allow for all kinds of interactions between the characters from both universes, like Sasuke getting involved in the power struggles in the Highschool DxD world.
The storylines can be combined by creating a common threat. Suppose an enemy emerges that is interested in both the power of the Bijuu and the power sources in Highschool DxD. This would force the characters from both universes to team up. Also, the cultural differences between the Naruto world and the Highschool DxD world can be explored. Naruto could introduce concepts like ninja clans and shinobi training to the DxD characters, while they could introduce him to the complex social hierarchies and supernatural politics in their world.
One way to combine them could be in a crossover universe. For example, Naruto could be summoned to the world of Highschool DxD through some sort of magical mishap. Gabriel, being an angel in her own right, could be involved in a plot to either help Naruto return home or to use his power for some greater purpose in the Highschool DxD world. Maybe Naruto's unique abilities, like his shadow clones or rasengan, could be seen as strange and powerful in the DxD world, and Gabriel might have insights on how to control or enhance them.
The Sharingan from Naruto could be combined with the enhanced vision abilities of some Highschool DxD characters. For example, if a character with the Sharingan teams up with a devil who has excellent vision for detecting enemies, they could create a super - surveillance team.
The clash of different power systems. In Naruto, there's chakra and various jutsu, while in Highschool DxD, there are sacred gears and demonic powers. Seeing how these interact, like if a devil tries to absorb chakra or a shinobi uses a sacred gear, would be exciting.
One way to combine them could be through a crossover event. Maybe some sort of interdimensional rift opens up and characters from Highschool DxD end up in the Justice League's universe. For example, Issei could find himself in Gotham City and have to team up with Batman to fight off a common enemy. Their different powers and abilities would make for an interesting dynamic. Issei's boosted gear and his ability to use dragon powers could complement Batman's detective skills and martial arts expertise.
Well, imagine if the Mind Stone was integrated into some sort of magical artifact in the Highschool DxD world. The characters might have to deal with the mind - controlling aspects of it. Issei could be affected by it, and his friends would have to find a way to break the hold while also dealing with the usual devils, angels, and fallen angels politics.
The chakra from Naruto's world and the Sacred Gears in Highschool DxD can be combined in various ways. For instance, a character could use chakra to enhance the power of a Sacred Gear. If a character has a Sacred Gear that gives them enhanced strength, they could use chakra to make that strength even greater, similar to how Naruto uses chakra to boost his physical abilities like the Rasengan.
In a Sharingan Naruto Highschool DxD fanfiction, the Sharingan could also play a role in terms of illusions. Imagine a scenario where the character uses Sharingan - based illusions to deceive the enemies from both worlds. They could make a Highschool DxD devil believe they are in a completely different environment like a dangerous Naruto - style forest filled with traps. And for Naruto characters, they could be made to see illusions of their worst nightmares or false threats from the Highschool DxD world, which would make for some really intense and engaging storylines.
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.
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.