Well, it could start with Naruto somehow ending up in the world of High School DxD. Rias might discover Naruto and see his potential, then make him her familiar. The plot could involve Naruto learning about the devil society and using his ninja skills to protect Rias and her peerage.
Definitely. The concept of combining different fictional universes in fanfiction is quite common. In a 'Naruto is Rias' Familiar' fanfiction, we could expect to see Naruto's ninja skills and powers being incorporated into the world of High School DxD. Maybe Naruto would use his shadow clone jutsu in battles in the DxD universe or his rasengan against some of the devils. It could also explore how Naruto, with his strong sense of justice and loyalty, would interact with Rias and the other characters in her peerage.
The plot could involve Naruto using his knowledge of chakra to diagnose Rias' brother's illness. He might discover that it's not a normal sickness but something related to the supernatural nature of their world. For example, his chakra flow could be disrupted by a dark magic spell.
Well, one possible plot could be that Naruto, as the son of Rias, discovers his unique heritage and has to deal with the expectations that come with it. He might struggle to balance his ninja skills with the devil - like powers he inherits from Rias. Maybe he uses his Naruto - like determination to bridge the gap between the ninja world and the world of devils.
Perhaps in this fanfiction, Naruto has lost his way in his own world and somehow ends up in Rias' realm. Rias, being kind - hearted, adopts him. Naruto would have to adapt to a whole new set of rules and powers. He might face challenges from other groups who are suspicious of this new addition, but with his ninja spirit, he overcomes them.
One possible plot could be that Naruto, as Rias's brother, brings his ninja skills to the world of High School DxD. He might use his jutsu to protect Rias and her peerage from various threats, like the devils from other factions or rogue exorcists. And along the way, he could also teach some of his ninja techniques to the members of Rias's peerage.
One possibility is that Naruto is summoned to Rias' world through some sort of magical ritual gone wrong. Once there, he forms an unlikely alliance with Rias. Naruto's chakra - based powers could be a great asset to Rias' team. They might start training together, with Naruto teaching the devils about chakra control and Rias showing Naruto some of the unique magic in her world.
One possible plot could be that Naruto and Rias, through some magical or unexpected circumstances, find themselves in a relationship. Their pregnancy could then bring about various challenges. For example, they might face opposition from their respective worlds. Naruto's world of ninjas might not understand his relationship with Rias from a different realm, and vice versa. Their friends and allies would have different reactions, some being supportive while others being skeptical. They would also have to deal with the practical aspects of preparing for a baby, like finding a safe place to live and ensuring the child's future in two very different and complex worlds.
In such a fanfiction, perhaps Naruto is in a new world where Rias, for some reason, completely ignores him. Maybe it could lead to Naruto feeling isolated and having to find his own way in this strange new place. He might start training alone and discover new powers out of his need to prove himself despite Rias's neglect.
One possible plot could be that Naruto, with his strong will and unique powers, enters the world where Rias exists. Their meeting might be accidental, perhaps during some sort of interdimensional rift. Naruto's sense of justice and Rias's leadership in her own group could lead to them having mutual respect. As they get to know each other more, they find that they share common goals in protecting the weak and maintaining balance. Eventually, they fall in love and decide to get married. Their union could bring about a new era of cooperation between the two different fictional worlds, with Naruto teaching some of his ninja techniques to Rias's group and Rias sharing her knowledge of magic with Naruto's friends.
They would likely start with some mutual curiosity. Naruto would be intrigued by Rias's devilish appearance and powers, while Rias would be interested in Naruto's ninja abilities. They might exchange stories about their respective worlds, with Naruto talking about his village and his friends, and Rias sharing about the devil society. Interaction could also involve sparring sessions to test each other's strength.