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.
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.
One interesting storyline could be Naruto and Moka teaming up to fight a common enemy. Since Naruto has the power of the Kyuubi, he could bring his combat skills and chakra control, while Moka, with her vampire abilities, could use her super strength and unique vampire powers like her Rosario. They might face a powerful demon or an evil organization that threatens both their worlds.
One common theme is the exploration of their combined powers. Since Naruto has the power of the Kyuubi and Moka has her own unique vampire abilities, stories might focus on how they use their powers in battles or to protect each other.
One interesting plotline could be Naruto gaining new and powerful abilities after the merge. For example, he might be able to access a new form of chakra that combines his own with the Kyuubi's. This could make him nearly unbeatable in battles against strong foes.
One interesting plotline could be Naruto gaining new and unique powers from the merge. For example, he might be able to access a special form of chakra that combines his own with the Kyuubi's, allowing him to perform super - strong jutsu. Another could be how his relationships with his friends and allies change. Maybe some are initially afraid of his new power, but he has to prove that he's still the same Naruto at heart.
One possible fanfiction could be about Naruto somehow ending up in the world of Highschool DxD and meeting Moka. Maybe Naruto's skills and his never - give - up attitude would impress Moka and the other characters. He could teach them some of his ninja techniques, like the Shadow Clone Jutsu, which could be useful in battles against the devils and other supernatural beings in the Highschool DxD world.
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.
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.
One interesting plotline could be Naruto and the good Kyuubi's merged form facing off against an ancient evil that threatens the entire ninja world. Their combined power gives them unique abilities to combat this threat.
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.