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 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.
In a Sharingan Naruto Highschool DxD fanfiction, the Sharingan could also play a role in terms of illusions. Imagine a scenario where the character uses Sharingan - based illusions to deceive the enemies from both worlds. They could make a Highschool DxD devil believe they are in a completely different environment like a dangerous Naruto - style forest filled with traps. And for Naruto characters, they could be made to see illusions of their worst nightmares or false threats from the Highschool DxD world, which would make for some really intense and engaging storylines.
If we consider Naruto and Kunou in a Highschool DxD fanfiction, Naruto could potentially be seen as a rival to some of the more powerful male characters in DxD. His determination and never - give - up attitude would be a contrast to the often more laid - back or self - interested characters in that world. Kunou, on the other hand, might form alliances with some of the female characters, sharing her knowledge of yokai with them and learning about the devils and angels in return.
One way is through the power system. The Susanoo's power could be compared or combined with the powers in Highschool DxD. Naruto could bring his ninja way of thinking into the Highschool DxD world. For instance, his idea of never giving up could inspire the Highschool DxD characters. And in terms of the world - building, the fanfic could have Naruto's world of ninjas and the Highschool DxD's world of demons and angels co - exist. The Susanoo could be seen as a new and fearsome power in the Highschool DxD world.
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.
A possible way for their worlds to merge in fanfiction is through a reincarnation plot. What if some of the Naruto characters were reincarnated in the Highschool DxD world? Naruto himself could be reborn as a devil or an angel. This would completely change the power balance and relationships in the Highschool DxD world. Rias would have to deal with this new Naruto, who still has his ninja spirit but now has to adapt to the rules and powers of the new world.
A great fanfiction could involve Naruto's overpowered (op) abilities being introduced into the Highschool DxD universe. For example, Naruto's shadow clone jutsu could be used in the battles at the high school in DxD. His ability to harness chakra might be a new form of power in that world.
One idea could be that Naruto characters attend the Highschool DxD academy. For example, Naruto himself could be a new student with his unique ninja skills. He might form an unlikely friendship with Issei. Another idea is a battle tournament where ninja techniques are pitted against the devilish powers in the DxD world.
Well, a cool plot might involve Issei from Highschool DxD being sent to the Naruto world. He could learn ninjutsu there and then bring some of those skills back to his own world. He might befriend Naruto and they could work together to fight a common enemy that has somehow crossed between the two worlds, like an evil group that wants to take over both the ninja world and the world of devils and angels.