Well, one way is to have a portal that connects the two worlds. Characters from both sides accidentally pass through it. Then they have to figure out how to get back while dealing with the problems in the other world.
Writers often start with a what - if scenario. What if Naruto was born in the Mass Effect universe? How would his life be different? They use this as a starting point to blend the storylines. They also consider the political aspects of both universes. The complex politics in Mass Effect like the council races and the ninja village politics in Naruto. By making characters interact with these political systems, they merge the two universes' storylines.
The blending could occur with the technology and powers. In Naruto, there are various jutsus and kekkei genkai. In Mass Effect, there's biotics and advanced technology. A fanfic could make it so that the shadowy elements in both worlds are related to a new form of power. For example, a shadow - based biotic power that is similar to the shadow - based ninjutsu in Naruto. This new power could be something that is being hunted or coveted by different groups in the story.
Well, a 'Naruto Godlike RWBY Fanfic' blends the two universes in multiple ways. Firstly, the story often has Naruto as an outsider who brings new knowledge and combat techniques. His use of chakra and elemental jutsus can be a huge advantage in the RWBY world. Secondly, the cultures are combined. The ninja culture from Naruto, with its clans and hierarchies, can interact with the RWBY culture of Huntsmen and Huntresses. This can lead to interesting scenarios where Naruto might train RWBY characters in ninja ways or vice - versa. Also, the fanfic might play with the idea of Naruto's godlike powers being a result of some sort of Remnant - related phenomenon, thus deeply connecting the two universes.
The characters from both universes interact in various ways. The harem aspect comes in as Naruto's charm and unique abilities attract characters from SAO. The fanfic might also blend the cultures of the two worlds. For instance, Naruto's respect for his comrades from the ninja world could influence how he forms relationships in the SAO harem. There could be training arcs where Naruto teaches the SAO characters some ninja techniques, or vice versa, the SAO characters introduce Naruto to new virtual world strategies. This creates a rich blend of the two universes.
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.
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.
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.
In many fanfics, the settings are combined. You could see the Hidden Leaf Village in the middle of Hyrule, or vice versa. The cultures also mix. The honor - based system of the ninja in Naruto could be applied to the characters in Zelda, or the magic system of Zelda could be introduced to the Naruto world. This creates a whole new and exciting world for the readers. It's really interesting to see how the different power dynamics, social structures, and technologies from both universes interact and change in these fanfics.
The storylines could be blended by having a shared goal. Suppose there is a threat that endangers both the Naruto world and the Sekirei world. It could be a new, powerful and evil force. Characters from both series would then have to work together to defeat this threat. Along the way, they would learn about each other's worlds, powers, and cultures. For instance, Naruto characters could teach Sekirei about teamwork in the context of a ninja squad, while Sekirei could show Naruto characters their unique way of bonding with their Ashikabi.
Typically, it blends the universes by having characters cross over. For example, Naruto might end up in the Star Wars galaxy or vice versa with Revan in the Naruto world. Their powers are then compared or combined. Naruto's chakra system could be related to the Force in some ways.