In this kind of fanfic, it might be that a group of rogue ninjas obtain devil breakers and start causing chaos in the ninja world. Naruto, being the hero he is, decides to study these devil breakers to find their weaknesses. He meets an old hermit who has some knowledge about them. Along the way, Naruto also has to deal with his friends' doubts about using such foreign and potentially dangerous technology. He has to prove that he can use it for good and stop the rogues before they destroy everything he holds dear.
It could be a rogue ninja who has mastered the devil breakers in a very dark way. He might use them to gain power and control over the ninja world, seeing Naruto as the main obstacle to his plans.
In a 'Naruto is a Phoenix Devil' fanfic, the plot could involve Naruto being transformed into a Phoenix Devil through some kind of magical or cursed event. This new form could bring internal struggles as he tries to control the devilish instincts while still holding onto his ninja values. He might also have to face off against other powerful entities in the ninja world who see his new power as a threat.
One possible plot could be that Naruto, in his devil form, meets Serafall in the underworld. They start off as rivals, constantly competing in various devilish tasks or battles. But as time goes on, they begin to see the good in each other. Naruto's strong sense of justice and loyalty, which he carried from his shinobi days, intrigues Serafall. And Serafall's charm and power attract Naruto. Eventually, they fall in love and get married, uniting the power of the shinobi world and the devil world in a unique way.
Well, perhaps Naruto's reincarnation as a devil would bring his ninja skills into the devilish realm. He might use his shadow clone jutsu in new and unexpected ways against devilish foes. Maybe he'd have to learn to control new devil powers while still holding onto his ninja values.
One idea could be a character with a devil bloodline being initially shunned in the Hidden Leaf Village but then proving their worth in a major battle. For example, they could use their unique devil - like abilities, such as enhanced speed and strength, to turn the tide against a powerful enemy.
One possible plot could be that Naruto discovers he has some devilish powers due to an ancient curse. He has to learn to control these powers while still maintaining his ninja way. He could face discrimination from his village at first, but then use his new powers to protect them from a powerful new enemy that normal ninja techniques can't defeat.
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 plot could be centered around a mysterious portal that opens up in the Forest of Death, connecting it to the world of Devil May Cry. Characters from both universes are drawn into this new situation. The ninjas are confused by the demonic entities they encounter, while the Devil May Cry characters are intrigued by the concept of ninjutsu. They must work together to close the portal before the two worlds collide catastrophically.
I'm not sure specifically as it could be many different things. It might be a fan - created story that combines elements of Naruto with some sort of super - devil concept. Maybe it features Naruto characters in a situation where they encounter a powerful devil - like being or gain devil - like powers.
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.