Yes, there are quite a few. Some focus on crossovers where Naruto's skills and experiences are combined with the world of DxD in interesting ways. For example, in some fanfictions, Naruto might use his Rasengan in battles against the devils in the DxD universe. You can find these on fanfiction platforms like FanFiction.net or Archive of Our Own.
Well, a frequent theme is the exploration of different cultures. The ninja culture from Naruto and the devil, angel, and other supernatural cultures in DxD collide. Writers often use this to create interesting stories. Also, redemption can be a theme. Maybe a character from DxD who was on the wrong path gets influenced by Naruto's positive attitude and tries to change for the better. And of course, there's always the theme of adventure as Naruto navigates this new and strange world.
Another plotline might be exploring Naruto's past more deeply. Maybe there are secrets about his parents that were never fully revealed in the original. In the fanfiction, he could discover something new about his father's time as Hokage or his mother's Uzumaki heritage that changes his perspective on his own powers and responsibilities.
In fanfiction, an older Naruto may have a more refined sense of leadership. He has seen more, experienced more battles and political situations. So, he might be more diplomatic in his approach compared to his younger self.
There probably is. A lot of fanfiction platforms are filled with creative crossovers. Given the popularity of Naruto and High School DxD, some fans might be intrigued by the idea of Naruto being in the Dragon DxD universe. They could explore how Naruto's abilities would interact with the unique magic and supernatural elements in the Dragon DxD world, like how his ninja skills would fare against the devils and angels in that setting.
One idea could be that Naruto is somehow transported to the world of Highschool DxD. He could use his ninja skills and chakra in the new world, maybe even teach some of the characters there a thing or two about combat. For example, he could show Issei how to better control his power using chakra control techniques.
Yes, there might be such fanfictions out there. Fanfiction writers often create crossovers where they put characters from different universes like Naruto and High School DxD together and come up with various scenarios, including ones where Naruto could potentially 'kill' a character from the DxD universe. However, the quality and nature of these fanfictions can vary greatly.
In a Naruto and High School DxD fanfiction, there could be a scenario where the protagonists of both stories meet in a neutral dimension. Issei from High School DxD might be intrigued by Naruto's ninja skills and determination. Naruto, in turn, could be curious about Issei's Sacred Gear. They could start training together, sharing their knowledge and skills, and ultimately have to face a common enemy that threatens both their worlds.
The storylines can be combined by creating a common threat. Suppose an enemy emerges that is interested in both the power of the Bijuu and the power sources in Highschool DxD. This would force the characters from both universes to team up. Also, the cultural differences between the Naruto world and the Highschool DxD world can be explored. Naruto could introduce concepts like ninja clans and shinobi training to the DxD characters, while they could introduce him to the complex social hierarchies and supernatural politics in their world.
The older brother in the fanfiction might be a character with a tragic past. For example, he could have been experimented on by Orochimaru and gained some strange powers as a result. And he returns to the village later to protect Naruto from the threats that are coming towards him.