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.
A 'Naruto Bijuu Highschool DxD' fanfiction might have some really cool crossovers. For example, the Bijuu could be seen as a new kind of power in the Highschool DxD universe. The students might be in awe or fear of Naruto and his connection to the powerful Bijuu. And Naruto, being the friendly guy he is, could try to use his powers for good in this new and different environment, like helping out in fights against the evil forces in the DxD 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.
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 female characters from Highschool DxD, like Rias, would be intrigued by the unique personalities of the Naruto characters. Rias might see potential in some of them to join her peerage. And the Naruto girls, like Sakura, would be a bit shocked at first by the more revealing outfits and flirty nature of some of the Highschool DxD characters. But over time, they would bond over things like protecting their friends and the school. The Naruto boys would respect the power of the Highschool DxD devils and might even try to incorporate some of their fighting strategies into their own ninja combat styles.
Naruto would probably be very friendly and curious. He'd be excited to meet all the different types of beings in the Highschool DxD world. He might start by trying to make friends with Issei, sharing his stories about his own battles and adventures in his world.
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.
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.
Often, the storylines blend by having characters from Naruto end up in the Highschool DxD world. For example, Naruto might be summoned there by some sort of magic accident. Then, he has to navigate this new world full of devils, angels, and other supernatural beings while still having his ninja mindset and skills.
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.
One possible storyline could be that Naruto, as a yokai, has to hide his true identity in the Highschool DxD world. He might get involved in the power struggles between different factions while also trying to fit in at school. Another could be that he uses his yokai powers to help out the main characters in Highschool DxD, like Issei, in battles against their enemies.