Obviously, Naruto would be one of the main characters. He's the protagonist of the Naruto series and would be the main link to the ninja world in this crossover. Another could be a young Kitsune, maybe one that's just coming of age in the Kitsune world. Their paths cross, and they have to learn to work together.
A Naruto and High School DxD crossover with kitsune elements could be really interesting. One aspect could be the introduction of Naruto's chakra system into the DxD universe. The kitsune in both series have unique powers. In Naruto, the nine - tailed fox is a powerful entity, and in High School DxD, kitsune also possess their own magic. A fanfiction could explore how a character from Naruto, perhaps one with kitsune - like abilities, interacts with the characters in the DxD world. They could share techniques or even form alliances. For example, Naruto characters could teach the DxD characters about chakra control, while the DxD characters introduce Naruto characters to the concept of sacred gears.
Maybe another kitsune who has gone bad. In the fanfiction world, there could be a rogue kitsune who sees Naruto as a threat. This antagonist kitsune might be jealous of Naruto's power or his connection to the human world. They could try to undermine Naruto's attempts to protect the village or his friends.
In this fanfiction, the idea of Naruto's transformation into a kitsune - like form could be really cool. It could be a physical transformation that gives him new powers and a new appearance. His speed, strength, and agility might all increase. And his relationship with other characters like Sasuke or Sakura could change as they adapt to his new identity as a kitsune.
One idea could be that Naruto discovers he has Kitsune blood in him. This could lead to him having new powers and a different connection to the spirit world. He might be able to communicate with Kitsune spirits and learn ancient Kitsune techniques.
A plotline could involve Naruto's Kitsune heritage being related to some ancient mystery in the Inuyasha world. For example, there could be a prophecy about a Kitsune from another world coming to save the Inuyasha world from a dark force. Naruto then has to discover his true role in this prophecy, all while dealing with the mistrust and different personalities of Inuyasha, Kagome, and the others. There could also be some romantic sub - plots, like Naruto developing feelings for one of the female characters in the Inuyasha world or vice versa.
To create a believable Kitsune character, start with its appearance. Give it the traditional features like multiple tails, which can signify its power level. For example, a young Kitsune might have only one tail, while an ancient and powerful one could have nine. Then, for its personality, combine traits from both Naruto and Fairytail universes. It could be as determined and loyal as the ninja in Naruto, but also have the fun - loving and adventurous spirit of the Fairytail mages.
One idea could be that Naruto discovers a hidden Kitsune realm. There, he meets a powerful Kitsune who offers to train him in unique chakra control techniques. This could lead to Naruto having new abilities and a different perspective on his ninja skills.
To create a believable plot, first, you need to understand the lore of both Kitsune and Naruto. For example, Kitsune are known for their trickery and magic. You could incorporate this into the story by having a Kitsune character play tricks on Naruto and his friends, but in a way that ties into the overall plot, like leading them to a hidden power or a secret enemy.
I'm not sure of specific fanfictions off the top of my head. But you can try searching on fanfiction - dedicated websites like Archive of Our Own or FanFiction.net. There, you can use the search filters to look for crossovers between Naruto and Inuyasha where Naruto is depicted as a Kitsune.