Well, perhaps Harry uses his magic to sense the One Ring's power. He might initially mistake it for a dark magical artifact from his own world. Then, he could start on a journey to figure out what this strange ring is, all while facing challenges from both the wizarding world (like Voldemort's followers thinking he has a new powerful weapon) and from Middle - earth forces that are also after the ring.
In this fanfiction, Harry could be transported to Middle - earth while chasing a dark wizard who has somehow gotten hold of the One Ring. Once there, he has to adapt to the new world and its rules. He uses his magic skills to help Frodo and his companions on their journey. Along the way, he might learn new forms of magic specific to Middle - earth and in turn, teach some of his wizarding skills to his new friends. This creates an interesting dynamic where the magic of two different fictional universes interact while they are all focused on keeping the One Ring out of the wrong hands.
Well, perhaps the One Ring is sent to Hogwarts as a mysterious artifact. Dumbledore, suspecting its great power and danger, enlists Harry to investigate. Harry, being the hero he is, soon realizes that the ring has a will of its own. He has to face the temptation of the ring while also dealing with the ongoing threat of Death Eaters, as the two evils start to intertwine in unexpected ways.
One possible plot could be that Harry stumbles upon the Sword of Gryffindor in a hidden chamber in the Forbidden Forest. Maybe he's on a solo quest to find a powerful weapon against Voldemort and the magic of the forest guides him to it. Then he has to figure out how to use its power to his advantage in the upcoming battles.
The snake could be an injured or lost magical creature. Harry, being the kind - hearted person he is, decides to take care of it. As he does, he realizes that the snake has a connection to a powerful wizard or witch who has been lost for ages. This connection then draws Harry into an adventure to find out what really happened to that powerful figure and how it affects the present state of the wizarding world.
Well, perhaps in this fanfiction, Harry Potter stumbles upon a lost baby in the wizarding world. He might then take it upon himself to find the baby's parents. Maybe he uses his magical skills, like casting spells to track the family. He could also seek help from his friends at Hogwarts, like Hermione who would surely be resourceful in finding clues about the baby's identity.
Well, perhaps in this fanfiction, Harry Potter stumbles upon a baby while on one of his adventures. Maybe the baby has some sort of magical power that he has to protect from the dark forces. It could also be that the baby is related to an old prophecy and Harry has to figure out how to keep it safe and fulfill the prophecy at the same time.
Definitely Voldemort and Sauron. Voldemort would see the One Ring as a powerful tool to conquer the wizarding world completely. Sauron, on the other hand, would not want anyone else to have control over the ring. So they would both be after Harry and the ring, making them the main antagonists.
A great plot could be that the ring is a portal - creating device. When activated, it opens portals to different parts of the wizarding world and even some unknown magical realms. A group of dark wizards is after the ring to use it for their evil plans. Harry and his friends are on a race against time to protect the ring and close the portals that the dark wizards have already opened, all while learning about the origin and true power of the ring.
The plot could be that the cat is a magical key to a hidden chamber. As a child, Harry stumbles upon the cat and realizes it has a strange mark. When he touches the mark, it reveals a map that leads him to a secret chamber filled with ancient magic and knowledge.
Perhaps Harry is transported to a different dimension where the Power Rangers exist. There, he discovers the Wolf Zord. He has to learn how to bond with it, just like the Rangers do. He could bring some of his magic skills to the table, and in turn, the Wolf Zord gives him new abilities to use against Voldemort or other foes back in his own world.
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.