Well, it could be about Naruto having some sort of divine powers inherited from Gabriel. Maybe he has to use these newfound powers to face tougher enemies in the Naruto world. He might also struggle with his identity as both a ninja and the son of a divine being.
One possible plot could be Harry growing up in a world filled with magic and mystery, given Gabriel's likely otherworldly nature. He might discover his powers gradually and struggle with the expectations that come with being Gabriel's son. Maybe he has to face off against dark forces that threaten his family and the world he knows.
One possible plot could be that Naruto is somehow transported to the Highschool DxD universe and meets Gabriel. They might team up to face a common threat that is endangering both the ninja world and the supernatural world of Highschool DxD. Naruto's unique skills as a ninja could be combined with Gabriel's angelic powers in an epic battle against evil forces.
Perhaps Naruto is mistaken for a powerful devil or an unknown being in the DxD universe. Gabriel, being curious, approaches him. They start off on a rocky relationship as Naruto is confused by this new world. But as they get to know each other, they find out that their powers can be combined in unique ways. For example, Naruto's shadow clone jutsu combined with Gabriel's holy powers could create some really cool and unexpected attacks against their foes.
Obviously Naruto as the main character. Gabriel would be a key figure, perhaps appearing in flashbacks or even in a more present form to guide Naruto. There could also be other Naruto characters like Kakashi, who might be intrigued by Naruto's new heritage and try to help him understand and control it.
In a 'naruto son of bellatrix fanfiction', perhaps Naruto has some of Bellatrix's wild and unpredictable nature. He could be on a journey to discover his true identity and how he came to be Bellatrix's son. This might lead him to explore the secrets of the wizarding world and his own ninja abilities in a new light.
Well, one possibility could be that Naruto inherits some of Gilgamesh's arrogance along with his power. Maybe he has a special ability to summon ancient Babylonian weapons. And he could use this in his battles in the Naruto world, like against the Akatsuki.
One possible plot could be that Naruto discovers his true heritage as the son of a princess from a hidden kingdom. He then has to deal with the politics and expectations of the royal family while still maintaining his ninja ways. Maybe he has to protect the kingdom from a new threat using his ninja skills.
In such a fanfiction, perhaps Naruto could be shunned by the village from the start. His 'devil' heritage might give him strange powers that he has to hide. He could struggle with self - identity, different from the Naruto in the original who is seen as a hero. Maybe he has to fight against the prejudice of the people who believe the devil's son can only bring destruction.
Well, it could be about Naruto having powers related to Hades on top of his ninja skills. Maybe he can control the dead or has some sort of underworld - like abilities that he uses in battles against the Akatsuki. He might also have to deal with the stigma of being the son of Hades in a world that doesn't understand such a thing.
Maybe Naruto is raised in Gotham instead of Konoha. He could inherit some of Catwoman's skills like stealth and agility, while still having his own ninja powers. He might fight crime in Gotham using a mix of his ninja techniques and Catwoman - like thievery skills, but for good causes.