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 possibility is that a writer might create a backstory where Naruto's unique chakra or abilities are needed to save Kunou's mother from a great danger. In the process, a bond forms between them and through some unorthodox means, Naruto becomes the father. Another aspect could be that in the fanfiction world, there are new rules and the author wants to explore the combination of Naruto's ninja world elements with the devil - based world of 'Highschool DxD' to make Naruto Kunou's father.
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.
If Naruto is Kunou's father, it would mean a whole new set of powers for Kunou. Combining Naruto's chakra - based powers with the devil - like abilities in 'Highschool DxD', Kunou could be extremely powerful. For example, she might be able to use Naruto's Rasengan technique along with her own devil powers.
Another possibility is that there is a tournament that spans multiple universes. Naruto is selected to represent his world, and Rias volunteers or is chosen to represent the High School DxD world. They meet at the tournament grounds. At first, they might be a bit suspicious of each other, but as they see each other's skills and determination in the battles, they start to respect and eventually become friends, working together to win the tournament for their respective worlds.
Well, if Naruto's world and the world of Danmachi with Freya collided, it could be really chaotic. Naruto's ninja skills like shadow clones and rasengan would be quite a shock to the adventurers in Danmachi. And Freya, with her charm and divine powers, might see Naruto as an interesting new being. Maybe Naruto would be mistaken for a powerful adventurer at first, and his determination to protect and his never - give - up attitude could inspire the people in the Danmachi world.
Their worlds could collide in a very chaotic yet exciting way. Naruto's flashy jutsu might shock the more down - to - earth military folks in Olivier's world. And Olivier's strict military leadership could be a new concept for Naruto to deal with.
There could be some really interesting interactions. Naruto might befriend Issei, and they could exchange training techniques. Naruto's rasengan could be a very unique move for Issei to learn about. Kunou could interact with Rias, perhaps having a bit of a rivalry at first due to their different leadership styles. Kunou is more reserved in her kitsune ways while Rias is bold as a devil leader.
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.
The female characters from Highschool DxD, like Rias, would be intrigued by the unique personalities of the Naruto characters. Rias might see potential in some of them to join her peerage. And the Naruto girls, like Sakura, would be a bit shocked at first by the more revealing outfits and flirty nature of some of the Highschool DxD characters. But over time, they would bond over things like protecting their friends and the school. The Naruto boys would respect the power of the Highschool DxD devils and might even try to incorporate some of their fighting strategies into their own ninja combat styles.
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.