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.
Perhaps Naruto, being Rias's brother, has to deal with the politics of the devil world. He could be a bit of an outlier with his ninja - like abilities and different moral code. Maybe he tries to reform the way the devils operate in the underworld, all while trying to keep his sister Rias safe. He might also have to face some challenges from the angels and fallen angels who don't trust his motives.
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.
The plot might involve Naruto trying to adapt to the new environment of being related to Rias. Since he has his own strong sense of justice from his ninja world, he could conflict with some of the more underhanded tactics in Rias' world. But in the end, they work together to overcome difficulties. For example, when facing a powerful enemy that neither of them could handle alone, they combine Naruto's ninjutsu and Rias' devil powers to defeat it.
Well, it could be that Naruto somehow gets transported to the world of High School DxD and discovers he has a connection to Rias, like being her long - lost brother. Maybe he uses his ninja skills to protect her from the various threats in her world.
The plot could involve Naruto finding himself in a situation where he has to interact with Rias's brother. The brother, being too involved in his own world of power and politics, simply brushes Naruto off as unimportant. Naruto then decides to prove himself not through grand displays of power, but through his kind heart. He helps those in Rias's group who are being overlooked or underestimated, just like he was in his own village. This catches the attention of Rias's brother who starts to see the value in Naruto he had previously neglected.
Well, one possible plot could be Naruto growing up in the world where Rias lives. He might struggle to fit in at first because of his different upbringing in the Naruto universe. But then, he could use his ninja skills and strong will to protect his sister Rias and her friends. He could also introduce some new ninja - like elements to their world, like new combat techniques or ways of training.
Well, in a 'Naruto is Rias' brother' fanfiction, there would be a lot of potential for character development. Naruto, being from a world where he had to fight for his place and for the safety of his village, would bring that same determination to protect Rias and her friends. He might be shocked at first by the devil society's hierarchy and customs, but would gradually find ways to fit in and make changes. Rias, on the other hand, would have a powerful and loyal brother by her side. They could face off against common enemies, like other devils who oppose Rias' ideas or angels who see Naruto as an unknown and dangerous element. The fanfiction could also explore their family backstory, how they were separated in the first place and what it means for them to be reunited now.
One possible fanfiction idea could be that Rias' sick brother is a key factor in a power struggle within their supernatural world. Naruto, being an outsider with his own set of skills and values, could get involved. Maybe he forms an alliance with Rias to protect her brother from those who would use his illness to their advantage.
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.
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.