One interesting fanfiction could be one where Naruto has werewolf - like abilities in the Highschool DxD universe. Maybe he uses his ninja skills along with his new werewolf powers to navigate the complex world of Highschool DxD. For example, he could use his enhanced senses from the werewolf side to detect hidden enemies in the school, and his ninja jutsus to fight them off. Another could be a story where the characters from both series cross paths in a unique way, like Naruto being mistaken for a new type of devil or something similar.
A creative plot could be Naruto being a werewolf who is sent to the Highschool DxD world to stop an impending doom that only his unique combination of abilities can prevent. Maybe there's a prophecy in the Highschool DxD world about a being with both ninja skills and werewolf powers that will save them all. Another plot could be that Naruto's werewolf form is the result of a curse from an ancient devil in the Highschool DxD world, and he has to find a way to break the curse while also dealing with the power struggles in that world.
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.
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.
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.
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.
A fanfiction idea could be that Lucifer from the Highschool DxD universe somehow ends up in the Naruto world. In the Naruto world, he could be seen as a powerful and mysterious figure. He might interact with the ninja clans and shinobi, and his powers could be both feared and respected. He could also have some kind of connection with the tailed beasts or the Sage of Six Paths. The story could revolve around how his presence disrupts or changes the balance of power in the Naruto world.
The appeal lies in the mash - up of different elements. It's like a buffet of cool concepts. You get the action - packed ninja world of Naruto, the unique sword powers of Zanpakuto, and the supernatural high school setting of Highschool DxD. It's a chance to see your favorite characters in new and unexpected situations.
A more complex fanfiction could be about a joint mission between the two universes. The leaders of both worlds realize that there is a common threat that only the combined forces of Highschool DxD and Naruto can defeat. So, they send their best characters, like Rias and Sasuke, to work together. This would involve a lot of character development as they learn about each other's cultures and powers.
One idea could be a crossover where Naruto characters are transported to the world of The Highschool of DxD. For example, Naruto himself could end up at the high school and his ninja skills would be quite a shock to the devils and other beings there. He might get involved in the Rating Games, using his shadow clones and other jutsu to his advantage.