It could be a new character created specifically for the fanfic. Maybe a powerful and malevolent spirit that has been lurking in the shadows of the Naruto world.
The devil could be a new, mysterious character created specifically for the fanfiction. It might be an ancient evil entity that has been lurking in the shadows of the Naruto world for ages.
The powers of devil fruits can be integrated by having characters discover them in a strange new land that has a connection to both the Naruto and One Piece universes. For example, a portal could open up and Naruto and his friends find a devil fruit tree. They then have to figure out how these new powers work within the context of their own ninja abilities. Maybe the devil fruit powers can be combined with chakra to create even more powerful attacks.
He could eat the Pika Pika no Mi (Glint - Glint Fruit) which gives the power of light, allowing him to move at the speed of light and create powerful light - based attacks. And perhaps the Yami Yami no Mi (Darkness - Darkness Fruit) which can absorb other attacks and create a gravitational pull. Combining the powers of light and darkness would be quite a unique and powerful combination for Naruto in this fanfiction.
Yes, there are likely some fanfictions out there with such themes. You can try searching on popular fanfiction websites like FanFiction.net or Archive of Our Own. Just type in 'godlike naruto devil' in the search bar and see what comes up.
If Naruto has devil wings in the fanfiction, they could enhance his physical strength. He could use the wings to generate strong gusts of wind during combat, either to blow away opponents or to create a diversion. Also, the wings might have some sort of defensive ability, like being able to harden and block incoming attacks. This would change his fighting style to be more aggressive and versatile, as he now has new ways to both attack and defend.
If Naruto ate a Paramecia Devil Fruit, his physical abilities might change drastically. For example, if it was a rubber - like Paramecia fruit, his already acrobatic fighting style could become even more so. He could stretch his limbs to attack from unexpected angles, much like Luffy in One Piece. This would give him an edge in close - combat situations, and his opponents would have a hard time predicting his moves.
The Naruto world has a rich lore with tailed beasts and different bloodlines. The Devil May Cry world has its own lore regarding demons and the underworld. In fanfictions that combine these two, the merging of these lores can result in really unique storylines. For example, a fanfiction could explore what would happen if a tailed beast was possessed by a powerful demon from the Devil May Cry world, and how the characters from both universes would deal with such a situation.
If Naruto had the Fire Devil Fruit, his combat style would change drastically. He could combine his existing ninja skills with the power of the fruit. For example, he might be able to infuse his Rasengan with fire, making it even more powerful. Also, his mobility could be enhanced as he could use fire to propel himself in new ways. This would make him a more formidable opponent in battles against strong foes like Sasuke or Pain.
One possible aspect of 'Naruto Devil King Fanfiction' could be the introduction of devil - like powers into the Naruto world. This could lead to really interesting character developments. For example, Naruto himself might have to struggle with new, darker forces within him, perhaps related to the Devil King's influence. It could also change the power dynamics between the different villages, as characters with devil - like abilities might pose a new threat or form new alliances.