Well, in such a fanfiction, there would likely be a mix of the Naruto universe's ninja elements and the High School DxD universe's supernatural elements. Naruto, as Akeno's brother, would probably be protective of her. They might get into all sorts of trouble, like Naruto accidentally revealing his ninja powers in Akeno's school and causing chaos. And then they'd have to work together to fix the mess while also dealing with any threats from the devil or other supernatural beings in that world.