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.
Character growth is also a major theme. Both Naruto and the characters from DxD have room for growth. In these fanfictions, they often learn from each other. For instance, Naruto might learn about the more complex social hierarchies in the DxD world and grow as a person, while DxD characters could learn about Naruto's perseverance and willpower.
Naruto characters could fit in by bringing their unique abilities. For instance, Naruto's rasengan could be a new and powerful attack in the Highschool DxD world. And in Moka's world, his ability to make friends and inspire loyalty could be a big plus. His ninja skills would also be useful for dealing with the various threats in both worlds.
A really interesting fanfiction could be where Naruto's Nine - Tails chakra has an unexpected reaction when he is near Moka. Maybe it flares up in a way that he can't control at first, and Moka's vampire instincts sense the power. This could lead to a situation where they have to work together to figure out how to control this new interaction between their powers. Also, Naruto could introduce the concept of ninja teams to the characters in Highschool DxD, which could change the way they approach battles and group activities.
One idea could be that Naruto is somehow transported to the world of High School DxD and meets Moka. He could use his ninja skills in the battles that take place in the DxD world. Maybe he teams up with Moka to fight off common enemies.
There's also 'Moka and Naruto's Adventure'. In this fanfiction, Moka meets Naruto and they embark on a journey together. It's full of action scenes as they face various enemies using their respective abilities. The author does a great job of making their interactions seem natural, and it's a fun read if you like crossovers.
One interesting crossover could be the combination of Naruto's ninja skills with the supernatural elements of Highschool DxD. For example, Naruto's shadow clone jutsu could be used in the battles against the devils and fallen angels in the Highschool DxD world. And Moka from Rosario + Vampire (I assume that's what you mean by Moka) could use her vampire powers in new and creative ways when interacting with Naruto's characters. Maybe she could use her strength to compete with Naruto's taijutsu experts.
Naruto's ability to use chakra could be a game - changer. His shadow clone jutsu could be used for various strategies, like in battles against the devils in High School DxD. When it comes to Moka, Naruto could use his clones to distract her while he tries to understand her vampire powers.
One idea could be a crossover adventure where Naruto and Moka find themselves in a strange new world together. They have to combine their unique abilities, Naruto's ninja skills and Moka's vampire powers, to survive and find a way back home.
Inner Moka is also likely to be in these fanfictions. Her more aggressive and powerful side compared to outer Moka can create different scenarios. For example, she might be initially hostile to Naruto but then gradually warm up to him as she sees his good qualities.