Maybe Naruto, as a fallen angel, has lost his powers and is trying to regain them while still in the Ninja world. He could be facing discrimination from both the angels (if they exist in the story's universe) and the Ninja who see him as something different.
In a 'Naruto is a Fallen Angel' fanfiction, the plot could be that Naruto was cast out of the angelic realm for some reason, perhaps for trying to protect the Ninja world which goes against the angelic laws. Now, he has to find a way to redeem himself. Along the way, he meets his old Ninja friends who don't recognize him at first, but gradually they come to accept him and help him in his quest for redemption.
The plot could involve Naruto being a fallen angel who has a special mission on Earth, which is intertwined with the Naruto - style conflicts in the Hidden Villages. For example, he might be there to stop an evil force that is related to the dark side of the angelic realm, and his Ninja skills and his latent angel powers are both needed to succeed.
Well, one possible plot could be that Naruto is cursed by some dark force and turns into a fallen angel. He might struggle with his new identity, as it goes against his usual positive and energetic self. His friends in Konoha could initially be scared or confused by his transformation, and Naruto has to prove that he's still the same person deep down while also dealing with the new powers and temptations that come with being a fallen angel.
One possible plot twist could be that Naruto's new fallen angel powers make him an outcast in Konoha. His friends might be afraid of him at first. For example, Sasuke could see him as a threat to his own power. Another twist could be that Naruto has to fight against a dark force that is trying to control his fallen angel powers for evil purposes. He would have to struggle to keep his good nature while having these new, potentially dangerous powers.
Well, perhaps Percy discovers that he has some hidden angelic heritage all along. But due to some misdeed or a curse, he becomes a fallen angel. His friends from the Percy Jackson world might not recognize him or might be afraid of him at first. He has to prove that he's still the same Percy on the inside while dealing with new angel - like powers that he doesn't fully understand. He could also be on a quest to redeem himself in the eyes of the angelic realm, which would involve facing powerful angelic enemies and solving ancient angelic mysteries. This would be a great way to blend the two different mythologies of Percy Jackson and the concept of fallen angels.
It could be an original character created specifically for the fanfiction. Maybe it's a being from a different dimension that somehow ended up in the Naruto universe. It might have some special powers or a tragic backstory that led to it being in conflict with Naruto.
One reason could be that it's simply not his genre. Naruto is used to his own action - packed, ninja - themed story. Fallen angel fanfiction is likely to be more on the supernatural and spiritual side, which he may not be interested in.
Maybe some ancient demonic clans that have a long - standing feud with the angels. They sense Naruto's transformation into a fallen angel and see him as an easy target or a way to get back at the angelic realm. These clans could have their own unique powers and abilities that Naruto would have to face.
In the 'naruto is an angel fanfiction', it could be that Naruto's angel form is a result of some ancient curse or blessing. His friends might be initially shocked but then come to rely on his newfound angelic abilities. He could use his powers to purify the darkness that lurks in the hearts of some of the villains in the Naruto universe, like Orochimaru. He might also be able to create shields of light to protect his village from attacks.
One idea could be that a fallen angel is sent to the Naruto world to retrieve a powerful artifact that has been lost there. Naruto, being the curious and heroic type, gets involved and they end up having to team up. Another could be that the fallen angel is on the run from some other celestial beings and Naruto offers her protection in his village. A third idea is that the fallen angel has some unique powers that Naruto wants to learn from to become stronger in his fight against the Akatsuki.
Maybe dxd is interested in exploring alternative storylines in the Naruto universe. The fallen angel aspect could add a new and exciting dimension to the familiar Naruto setting. It could also be that dxd enjoys the creativity of fanfiction writers in general and stumbled upon this Naruto - fallen angel one and got hooked.
Well, perhaps Naruto could be sent to protect a certain village or person. He might use his ninja skills in a new way to fend off threats from the spirit world or other supernatural forces.