In a fanfic, the merging could also be due to a common threat. Suppose there is a dark force that wants to conquer both the ninja world and the vampire world. To fight this threat, Naruto and Moka's worlds start to merge gradually. This would result in unique alliances. For example, ninja and vampires might have to work together, with Naruto and Moka leading the way. They would need to find a way to combine their different fighting styles and powers, like Naruto's shadow clones and Moka's blood-based attacks.
Another way their worlds could merge is through a time - space distortion. Maybe a powerful jutsu gone wrong or a vampire ritual that backfires causes the two worlds to start overlapping. As a result, Naruto and Moka find themselves in a strange new world that is a mix of both their original worlds. They would then have to explore this new world, deal with the new rules and powers that emerge from the merge, and also face the challenges that come with it, such as new enemies that are created from the combination of ninja and vampire powers.
Their worlds could merge through a magical portal. Maybe a powerful jutsu in Naruto's world accidentally creates a rift that connects to Bayonetta's dimension. This would allow characters from both sides to interact.
In a Naruto and Batman crossover fanfiction, their worlds could merge in a few ways. One way could be through a portal. Maybe some sort of evil magic or advanced technology creates a portal between Konoha and Gotham. This would bring Naruto and his friends into Batman's world, or vice versa. Another possibility is that a powerful villain has the ability to manipulate space - time and decides to bring the two worlds together for his own evil purposes.
The worlds could merge through a mysterious portal. This portal could be created by some powerful jutsu in Naruto's world that goes wrong or by an experiment in Build's world. Once the worlds start to merge, the characters would be in for a shock. The ninjas in Naruto's world would see the advanced technology of Build's world, and the people in Build's world would be amazed by the use of chakra.
Naruto's character becomes more complex. He gains a new level of confidence due to the power of the merge. He also becomes more aware of his own strength and how to use it responsibly.
In a merged world, the power structures would change. The Gems, who are used to being the protectors of Earth in their world, would now have to share that role with Ben. Ben's aliens could provide new ways to defend the planet, while the Gems' magic could offer solutions to problems that Ben's aliens couldn't handle alone. Also, the villains from both worlds might team up, forcing Ben and Steven to form a united front. They would need to combine their powers, like Ben using an alien form that could enhance Steven's powers, or Steven using his powers to boost an alien's abilities.
The merge can also happen on a more conceptual level. For example, the idea of friendship and teamwork, which are important in both series, could be the connection. Naruto's belief in his friends and the Digidestined's bond with their Digimon could be seen as similar. This similarity could be the starting point for the two worlds to interact in fanfiction. Writers might have Naruto and the Digidestined team up to fight a common enemy that threatens both the Digital World and the ninja world. They would combine their unique powers and skills to achieve victory.
Yes, there are some out there. You can start by looking on fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'kyuubi naruto x moka' and you'll likely find a variety of stories with different themes and plotlines.
For Naruto's world, his relationship with Inner Moka could cause some initial unrest. But as people see how much they love each other, it could lead to a more inclusive society. The Leaf Village could start trading knowledge with the vampire world. In Inner Moka's world, her marriage to Naruto might inspire younger vampires to be more adventurous and look beyond their own kind for relationships. It could also lead to a cultural exchange, with vampire traditions being shared in the ninja world and vice versa.
His relationship with Sasuke could change significantly. With the new power from the merge, Naruto might be seen as more of a threat by Sasuke, or Sasuke could be jealous of Naruto's newfound strength. On the other hand, it could also lead to Sasuke respecting Naruto more and seeing him as an equal in a different way.
Their worlds could merge in a lot of ways. For instance, the technology from the Avengers' world could be introduced to Lilo and Stitch's Hawaiian setting. Maybe Stark Industries could set up a research facility in Hawaii to study Stitch's biology. And in return, Lilo could introduce the Avengers to the local culture and the concept of 'ohana', which could strengthen the Avengers' sense of family within the team.