It could be Naruto himself. He has a strong personality and a lot of potential for growth in a new world like DxD. His journey to adapt to the DxD universe and its rules would be very engaging.
Perhaps a new character created specifically for this fanfic could be the main character. This character could be a hybrid of Naruto and DxD elements, having the ability to use both ninjutsu and sacred gears. They would have a unique perspective and could bridge the gap between the two universes.
In this fanfic, the power - up concepts can be very interesting. Since both Naruto and DxD have different power systems. A character might gain new powers by combining the chakra system with the sacred gears in DxD. This could lead to unique abilities that we haven't seen in either of the original stories, like a sage - mode enhanced by a sacred gear's power.
Naruto and Rias could be a great pairing. Naruto's positive and determined nature would complement Rias' leadership and strong - willed personality. They could work together to protect their respective groups and also learn from each other's powers and cultures.
Naruto himself could be the main character in a Shinju fanfic. He has always been about finding new ways to bring peace and understanding. His connection to the nine - tails could play an interesting role if he were to interact with the Shinju. Maybe he could try to convince the Shinju that the ninja world has changed and it doesn't need to be a source of fear and destruction.
Sasuke Uchiha could be a great choice as the main character. He already has a complex backstory and a lot of room for character development. His journey from seeking revenge to finding his own path in the Naruto series can be further explored in a fanfic.
It could be an original character. Maybe a young ninja who discovers their forging abilities by accident. They might come from a small, unassuming clan that has kept the forging secrets hidden for generations. This character would then have to navigate the Naruto world while keeping their newfound power a secret at first, until they realize they can use it to make a difference.
Perhaps Sasuke could be the main character in a different take on a 'hitman naruto fanfic'. He already has his own vendetta and his skills as a ninja could easily be translated into those of a hitman. He has the Sharingan, which gives him an advantage in combat and tracking, making him a formidable assassin - like figure.
It could be a new character that you create specifically for the fanfic. This character could be an outcast from their village because of their strange Koton abilities. They then have to prove themselves and their powers to the others.
Sasuke could also be a main character in such a fanfic. Given his quest for power and his complex personality, his encounter with the Shinigami could lead to some really interesting character development. He might be tempted by the Shinigami's power or be forced to make difficult choices that would change his path.
Rock Lee could be an excellent main character for such a fanfic. Since he already focuses on physical combat in the form of taijutsu, wushu would be a natural extension for him. He could be sent on a journey to learn the true essence of wushu and bring that knowledge back to Konoha to revolutionize the way they think about physical combat.
Sasuke could be a great main character for a swordsman - themed fanfic. Since he already uses a sword - like weapon, the story could focus on his further development as a swordsman, exploring different sword techniques and their origins.