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.
A great way to blend these universes in fanfiction is to focus on the power structures. In Naruto, there are different levels of ninja abilities and in Highschool DxD, there are the ranks among devils. Leviathan, being a powerful being in Highschool DxD, could be equivalent to a Kage - level character in Naruto in terms of power. A story could be about a young devil from Highschool DxD training with Naruto characters to reach a new level of power, with Leviathan as a sort of mentor or goal to reach in terms of strength.
To blend these universes, consider the cultural aspects. Naruto has its own Japanese - inspired culture with things like the importance of honor and clans. Highschool DxD has a more Western - influenced devilish culture. Leviathan, as a part of the Highschool DxD universe, could be used to bridge these cultures. For example, a fanfiction could tell the story of a Naruto character trying to understand the devilish society in Highschool DxD with Leviathan as a guide, and in the process, they introduce elements of their own ninja culture to the devils.
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.
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.
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 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.
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 interesting crossover could be a story where the power - up systems of Naruto and Highschool DxD are combined. For example, a character from Highschool DxD might learn how to use chakra like in Naruto. And Leviathan could be a powerful entity in this new world, perhaps with abilities influenced by both Naruto's jutsu and the unique powers in Highschool DxD. This could lead to really cool battle scenes and character development.
One idea could be a crossover where Naruto characters end up in the Highschool DxD world and have to deal with the devils and angels there. For example, Naruto could use his shadow clone jutsu in a battle against a group of devils, creating chaos. Another plot idea might be that a Leviathan character from Highschool DxD travels to the Naruto world and tries to learn ninjutsu. It could lead to some comical situations as they struggle with chakra control.
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.
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.
Fanfiction writers blend the universes by creating new storylines. They might have Naruto end up in RWBY's world due to some magical accident. Once there, he has to deal with Salem's threat. The RWBY characters are initially skeptical of Naruto but soon come to respect his abilities. The different cultures of the universes are also merged. The shinobi way of life from Naruto can be seen rubbing off on the RWBY characters, and the technology and magic of RWBY can be something new and exciting for Naruto. Salem, as a powerful antagonist, adds an extra layer of danger and mystery to the blend of these universes.