Well, it could be about a dark version of Naruto having different goals and values from the original Naruto. Maybe the black Naruto wants to take over the Hidden Leaf Village by any means, while the real Naruto tries to stop him to protect his home and friends.
One possible plot could be that the black Naruto is a result of Naruto's repressed negative emotions. The black Naruto starts causing chaos in the ninja world, and Naruto has to fight him. During the fight, Naruto realizes that he needs to face his own inner demons. He discovers that the black Naruto represents all the things he has been afraid of becoming. By accepting and confronting these aspects of himself, Naruto is able to defeat the black Naruto and become an even stronger and more self - aware ninja.
Maybe in the fanfic, there is a dark force that tries to manipulate Hiashi into attacking Naruto. The fanfic could explore how Naruto tries to reach out to Hiashi, to show him that they are being played. Along the way, they have an intense battle where Naruto shows off his new jutsus and Hiashi uses his clan's most secret techniques. In the end, they realize they were on the same side all along.
A fanfic plot could be that Acrpnolgia was once a powerful ninja in Naruto's world but was banished long ago due to his dangerous and unethical experiments. He returns now with a grudge and enhanced powers. Naruto is tasked with stopping him as he is seen as the hope of the village. During the battle, Naruto tries to understand Acrpnolgia's motives and find a way to end the fight without having to kill him, but Acrpnolgia is so consumed by his hatred that it becomes a very difficult task for Naruto.
In a 'Naruto Monsters vs Aliens' fanfic, perhaps the aliens are trying to enslave the people of the Naruto world. The monsters, which were once feared by the villagers, now become unexpected allies. Naruto, with his belief in friendship and understanding, tries to bridge the gap between the two groups. He might train with the monsters to learn new combat techniques and in turn, teach them about the importance of unity. Together, they could fight against the aliens' mind - control devices and save the world.
The fanfic could be about a dimension - hopping adventure. Naruto accidentally ends up in Mizore's world. Their first encounter is hostile as they don't understand each other. Mizore attacks Naruto with her ice powers, thinking he's an intruder. Naruto retaliates with his ninja techniques. But as they interact more, they realize they have more in common than they thought. They could be on a quest to find a way to send Naruto back home while also dealing with threats in Mizore's world.
Well, it could be that Naruto uses his new Sage of Six Paths powers to combat the Juubi. Maybe he has to find a way to break through the Juubi's defenses which are super strong. He might also get some help from his friends at some point in the fanfic.
A plot might involve Shukaku being controlled by an evil force and Naruto has to fight it to save the village. Naruto starts by trying to reason with Shukaku, but when that fails, he has no choice but to engage in a fierce battle. He might call upon the help of his friends like Sasuke or Sakura at some point to create a strategy to defeat Shukaku.
Well, one possible plot could be that Naruto somehow gets transported to the DC universe and becomes a huge fan of Black Adam. He might see Black Adam's power and determination and be inspired to learn new jutsu or ways of fighting. Maybe he tries to incorporate some of Black Adam's battle tactics into his own style in the ninja world.
One possible plot could be that Naruto and his friends discover a strange new world where plants have to fight off zombies. Naruto, with his ninja skills, decides to help the plants. He could use his shadow clone jutsu to multiply and fend off waves of zombies. The plants, in turn, could offer Naruto and his team special powers like the ability to shoot fireballs from their mouths, just like some of the plants in the game do.
One possible plot twist could be that the real enemy is secretly manipulating the Shinobi Alliance to fight Naruto. So Naruto has to not only defend himself but also figure out who is behind it all. Another could be that Naruto discovers a hidden power within himself during the fight that changes the course of the battle.
It really depends on how the author writes it. If the black Naruto is written as being more powerful with no moral constraints, he might seem like the likely winner at first. But Naruto has the power of friendship and his strong will, so he could also come out on top.