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.
One way they can blend in fanfiction is through the characters' personalities. Saitama's laid - back yet extremely powerful personality could contrast with Naruto's determined and growth - oriented one. In Highschool DxD, the characters are often more flamboyant and passionate. You could write a story where they team up to fight a common enemy. Maybe an otherworldly threat that combines the threats from each of their original worlds. Saitama could use his one - punch to quickly take out some of the minions, while Naruto uses his Rasengan and other jutsu, and the Highschool DxD characters use their Sacred Gears and demonic powers.
Well, one way is through character exchanges. Imagine a character from Naruto like Naruto himself ending up in the Highschool DxD universe and meeting Leviathan. He could introduce his ninja skills to that world and also learn about the devil powers. Another way is to have a common enemy. For instance, a new threat that can only be defeated by combining the unique powers from all three, like the Rasengan combined with a Highschool DxD power against an enemy threatening Leviathan.
One way is through the power system. The Susanoo's power could be compared or combined with the powers in Highschool DxD. Naruto could bring his ninja way of thinking into the Highschool DxD world. For instance, his idea of never giving up could inspire the Highschool DxD characters. And in terms of the world - building, the fanfic could have Naruto's world of ninjas and the Highschool DxD's world of demons and angels co - exist. The Susanoo could be seen as a new and fearsome power in the Highschool DxD world.
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.
Another storyline could involve Sage Naruto teaching some of the characters in Highschool DxD his ninja techniques. Imagine Naruto showing Issei how to use shadow clones to his advantage in battles or training sessions. This would not only add a new dimension to the battles in the DxD world but also create some really fun and comical moments as the characters from two different universes interact.
Well, first of all, the writers often take the unique elements from each universe. From Naruto, it's the ninjutsu, kekkei genkai, and the concept of the ninja villages. From Highschool of the Dead, it's the zombie apocalypse and the school environment. They then find ways to make these elements work together. Maybe a character with a Sharingan from Naruto can use its powers to detect zombies from afar. Also, they might use the character relationships. A Naruto character could form a team with some Highschool of the Dead survivors to face the challenges together. And they might also play with the different values of the two universes. For example, the loyalty in Naruto and the survival instinct in Highschool of the Dead.
One way is through character crossovers. For example, a character from Fate Stay Night could end up in the Highschool DxD universe or vice versa. They could then interact with the native characters, creating new and interesting storylines. Another way is to combine the magic systems. The magic in Fate Stay Night is very different from the supernatural powers in Highschool DxD, and blending them could lead to unique and exciting plot developments.
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 fanfictions often add new jutsus or powers to the characters, thus expanding the power system of the universe. For instance, a story might introduce a new type of Sharingan ability. In Highschool DxD, fanfictions can expand on the idea of the Sacred Gears. A fanfic like 'The Sacred Gear Evolution' shows how these gears can have even more complex and powerful forms, which enriches the Highschool DxD universe.
A fanfiction might explore a story where the Occult Research Club members are all transported to a different dimension. In this new world, they have to find a way back home while facing new enemies and challenges. Rias could use her leadership skills in a different way, and Kiba might discover new powers within himself to protect the group. This would also test the bonds between the characters in a whole new setting.