It could be that their union was unexpected, and their pregnancy brings about a change in the power balance in their respective universes. For example, their child could be a new form of being with combined ninja and devil powers. The story might explore how they train and raise this child, while also dealing with the political and social implications of their relationship in both the ninja and devil communities. There could be those who oppose their relationship and try to harm them, and they have to fight back to protect their family.
In this fanfiction, perhaps Naruto's strong sense of protection kicks in. Since he is used to protecting his village, he now focuses on protecting Rias and their unborn child. He might use his ninja skills to ensure their safety while Rias uses her devil powers to support them in other ways, like gathering resources or information. And the story could revolve around their journey of preparing for the new life together.
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.
Maybe Naruto is on a mission to find a powerful artifact. He discovers that this artifact has some connection to the world of Rias and Akeno. When he arrives there, he meets them. Rias is initially suspicious of Naruto but Akeno is intrigued. As they work together to uncover the mystery of the artifact, Naruto's chakra starts to interact with their magic in unexpected ways, leading to new powers emerging among them and a growing friendship and more between the three.
Well, perhaps Naruto is on a quest in his own world and accidentally stumbles into the realm of the Rias family. The family could be intrigued by his unique chakra and Naruto could learn about their devil powers. They could then team up to face a common enemy that threatens both the ninja world and their devilish domain. Maybe a powerful demon that wants to take over both worlds.
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.
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.
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.
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, 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.
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.
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.