It would change it a great deal. The One Ring's nature is to dominate and control. In Naruto's world, that could mean a shift in the power balance. If Naruto got hold of the One Ring, he might initially think he could use it for good. But as time goes on, the Ring's power could seep into his psyche. His friends might start to notice a change in him, and his relationships could become strained. Also, other villains in the Naruto universe might try to steal the Ring from him, leading to a whole new set of battles and challenges.
Well, in such a fanfiction, Naruto might be really confused at first. The One Ring has its own dark power and lore from Middle - earth, while Naruto is from the ninja world. Maybe Naruto would think it's some kind of super - powerful ninja tool at first. He might try to use his chakra on it, not realizing the corrupting influence it could have. And his friends like Sasuke and Sakura would be very suspicious of this strange new item Naruto has found.
Hermione would be extremely worried. She'd start researching right away, trying to find out all she could about the One Ring. She might try to find spells to contain or protect against its power. As for Voldemort, he would see the One Ring as a tool to gain more power. He would likely be overconfident in his ability to control it, not realizing that the ring could potentially control him instead.
He could gain the ability to become invisible at will. The One Ring has the power of stealth in Tolkien's lore, and this could translate to Naruto being able to sneak up on his enemies undetected. He might also gain the power to influence the minds of others, which would be a huge advantage in negotiations or battles.
Perhaps the One Ring has some sort of connection to Voldemort. Harry, having the One Ring, finds that Voldemort is drawn to it because of its power. This could lead to a new battle where Harry not only has to face Voldemort but also has to decide what to do with the Ring. He might have to journey to find a way to destroy it, just like in the original 'Lord of the Rings' story.
One possibility is that Naruto would try to incorporate the ring into his ninja gear. He's always looking for ways to get stronger. But the ring's power is not like normal ninja power. It could start to corrupt him slowly. For example, he might become more obsessed with power and less concerned about his friends' feelings.
Sauron, even in the context of this fanfiction crossover, could be an antagonist. His influence could reach the wizarding world through the One Ring. He might try to manipulate Harry or those around him to get the Ring back.
Well, in 'naruto millennium ring fanfiction', it could potentially revolve around the idea that the millennium ring is an ancient artifact with great power. Naruto might come across it and then have to deal with the consequences. For example, there could be enemies who also want the ring for its power, and Naruto has to protect it while also figuring out what its true purpose is. This could lead to him learning new jutsu or forming unexpected partnerships with characters we don't usually see him with in the original series.
A common theme is power enhancement. The ring often gives Naruto or other characters some extraordinary power. Another theme is mystery. The origin and true nature of the ring are usually mysterious.
Obviously Naruto would be one of the main characters. There might also be his friends like Sasuke and Sakura. But since it's a fanfiction involving a special millennium ring, there could be new characters created specifically for this story who are also important.