A pairing between a Nephilim character and Kiba from Naruto could be interesting. The Nephilim's otherworldly nature and Kiba's loyalty and connection to his ninja dogs could create a unique dynamic. They could go on adventures together in the fanfiction, exploring the mysteries of both universes.
Another possible pairing is Issei from Highschool DxD and Sasuke from Naruto. Issei's pervy nature could be tempered by Sasuke's cool and collected demeanor. And in terms of power, they could learn from each other and grow stronger together, perhaps facing off against common enemies in the fanfiction.
One potential pairing could be Naruto and Rias. Naruto's determination and strong will would pair well with Rias' leadership and powerful devil abilities. They could support each other in battles and in understanding the differences between their two worlds.
One idea could be a crossover where Naruto characters end up in the Highschool DxD world and have to adapt to the different rules regarding magic and devils. For example, Naruto could use his chakra in unique ways against the devils' powers.
Another common pairing is Sasuke and Akeno. Sasuke's brooding and powerful nature might attract Akeno, who is also quite powerful and has a bit of a mysterious side herself. They could bond over their shared power levels and a mutual understanding of what it means to be different from the norm in their respective worlds.
Koneko and a god of nature could form a unique pairing. Koneko's connection to her cat - like instincts might be related to the god of nature's domain. The god could help Koneko further explore and enhance her powers, while Koneko could show the god a different perspective on the world of the supernatural that exists in the High School DxD universe.
Blake Belladonna and Koneko Toujou. Both are strong female characters with a bit of a mysterious side. Blake's Faunus heritage and her experience with the White Fang could be something that Koneko, with her Nekomata powers, would be interested in. They could bond over their similarities and differences, and perhaps team up in battles against common foes.
A potential plotline could be Naruto arriving in the Highschool DxD world and being mistaken for a new type of devil or other supernatural being due to his 'dark godlike' nature. He would then have to prove his intentions and find his place in this new world. Another plotline could involve Naruto using his skills to train the characters in Highschool DxD in combat, similar to how he trained his friends in the Naruto world.
Griselda and Issei could make an interesting pairing. Their different personalities could create some fun and dynamic interactions. Issei's boldness and Griselda's more reserved nature could be a contrast that works well in a story.
A potential plot twist could be that the Kyuubi has a hidden connection to an ancient power in the Highschool DxD universe. Maybe it was sealed away long ago by the devils or angels and Naruto's presence awakens it. Another twist could be that Naruto is initially thought to be a threat by the main characters of Highschool DxD, but it turns out he is the key to solving a major problem they are facing, like a powerful enemy that is immune to their usual forms of attack but is vulnerable to Naruto's jutsu.
A character like Geralt from The Witcher could be an interesting crossover. He has his own set of skills and a no - nonsense attitude. In a Highschool DxD Dragonborn fanfic, he could be brought in to help train the dragonborn or fight alongside them against common enemies.
Ruby and Issei would be an interesting pairing. Ruby's innocent yet determined nature combined with Issei's perverted but also heroic side could lead to some really fun and dynamic interactions. They could learn a lot from each other in terms of combat and dealing with different types of threats.
Ichigo and Akeno could also be an exciting pairing. Akeno has her own unique powers and a mysterious side. Ichigo's straightforwardness and strong moral code could contrast with Akeno's more complex nature. They could work together in battles, with Ichigo providing the brute force and Akeno using her strategic and powerful attacks.