What could be the plot if Naruto is Ravel's brother in fanfic?

No. In the official Naruto and High School DxD universes, there's no relation between them. But in fanfic, the writer can make up any relationship they want. It might be for the sake of adding an interesting twist to the story, like bringing together the ninja world of Naruto and the devil - based world of High School DxD in a new family - centered way.

A possible scenario is that Ravel teams up with Naruto to fight a common enemy. Ravel's magical abilities and Naruto's ninjutsu would make for an interesting combination. They could learn from each other's fighting styles during their battles, and their friendship could grow as they face challenges together.

It could be about Naruto having a special bond with Souna as his brother. Maybe they go on adventures together, face challenges in the ninja world side by side, and Naruto uses his skills to protect Souna from various threats.

Well, it could be about Naruto and Sona's brother having adventures together. Maybe they team up to face a common enemy in the ninja world. They could learn from each other's skills and abilities.

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.

One possible plot could be that Naruto, as Hashirama's brother, has a different upbringing. Maybe he was hidden away because of his 'godlike' powers. As the story progresses, he emerges to help the Hidden Leaf Village in a time of great need, using his powers to defeat powerful enemies that the village couldn't handle on its own.

One possible plot could be that Naruto grows up with a different understanding of the Uchiha clan because of his connection to Shisui. He might be more involved in Uchiha affairs from an early age. For instance, he could try to prevent the Uchiha massacre, using his and Shisui's combined influence.

Well, the plot could involve Naruto having some sort of hidden origin that links him to Madara as his brother. Maybe Naruto discovers this truth and has to deal with the implications. He could use his godlike powers to either unite or oppose the Uchiha clan, depending on how the story goes.

One possible fanfic could be about Ravel from Highschool DxD finding herself in the Naruto world. Maybe she could use her unique abilities, like her magic skills, to interact with the ninjas. For example, she might befriend Naruto and help him with a difficult mission using her magical powers to provide an unexpected advantage.

One possible plot could be that Naruto grows up knowing about his relation to Tsunade. He could be trained in the art of medical ninjutsu by Tsunade's family, and then use his unique Uzumaki chakra to enhance it. As Tsunade's older brother, he might be very protective of her, especially after the death of her loved ones. He could play a key role in helping Tsunade become the Hokage, perhaps by convincing the village elders that she is the right choice, using his own influence as a respected ninja.