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.
Fanfictions for both series also expand the universes by creating new story arcs. In Naruto, a fanfic could be set in a time skip where new villains emerge. In Highschool DxD, a story might take place in a different dimension within the DxD world. This gives the characters new challenges and environments to interact with, like in the Highschool DxD fanfic 'The Otherworldly Encounter' where the characters have to deal with threats from a parallel dimension.
These fanfictions expand the Highschool DxD universe by introducing new characters like male Nekoshou. For example, they can create new storylines around their powers and how they interact with the existing characters in the DxD world. It might be a male Nekoshou with a special ability that changes the power dynamics in the school.
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 possible blend in the fanfiction could be through a magical or technological device. Maybe a scientist in Highschool DxD creates a device that can open portals to other universes, and Naruto's world is one of the destinations. When Naruto arrives, he discovers that there are elements related to Beelzebub there. The device could malfunction and start to merge the three universes in unexpected ways. This would force the characters to adapt to the new situation, with Naruto using his ninja training to survive, the Highschool DxD characters using their social and magical skills, and those related to Beelzebub using their demonic powers to deal with the chaos.
One way could be through a powerful magic - ninjutsu hybrid spell. Maybe a character with knowledge of both worlds accidentally creates a portal that merges the two. For example, if a shinobi in Naruto world somehow gets hold of a magic item from Highschool DxD and activates it in a wrong way.
Some fanfictions expand on their relationship by introducing new conflicts that are more relationship - based. In the original, they have external threats. But in fanfictions, it could be something like a misunderstanding between them that tests their love. This forces them to communicate more and grow closer. In one fanfic, there was a situation where Issei was misunderstood to be interested in another girl, and it led to a really emotional journey for both of them to resolve the issue and strengthen their bond.
In some fanfictions, the merge happens when a powerful artifact from either universe is activated. This could be a Kamen Rider's belt or a powerful relic from Highschool DxD. When this happens, it creates a rift that pulls elements from one universe into the other. The characters then have to deal with the new and unexpected situations. For instance, the devils in DxD might be intrigued by the Kamen Rider technology and try to harness it for their own purposes.
Another approach could be a shared enemy. Suppose there is an entity that exists in both universes in a different form. Writers could create a story where the characters from Highschool DxD and Prototype have to team up to defeat this entity. This would require them to understand each other's powers and work together, leading to a lot of character development and exciting action scenes.
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.
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 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.