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.
One popular fanfiction could be 'Harry Potter and the Lost Kitten'. In this story, as a child, Harry discovers a magical cat that leads him on an adventure through the Forbidden Forest. It has elements of mystery and friendship.
Well, in the fanfiction, Harry might first try to find the owner of the cat. He could use magic to see if there are any identification spells on the cat. Then, he might take it to Hogwarts. The cat could be a magical cat with special abilities, like being able to talk or lead Harry to a hidden place in the school.
It might change Harry's perspective on magic. If the cat has some unique magical powers, Harry could learn a lot from it. For example, if the cat can transform into different animals, Harry might be inspired to study more about transfiguration magic. Also, taking care of the cat could teach Harry more about responsibility.
Perhaps all the main characters are transformed into cats at some point. Harry, Hermione, and Ron have to navigate the wizarding world as cats. They might use their cat - like abilities, such as stealth and agility, to solve mysteries and defeat dark wizards. The Hogwarts castle could also have new areas that are only accessible to cats, where they find clues and magical artifacts.
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.
It could be a snowy owl, just like Hedwig. In the original series, Hedwig was Harry's loyal and beautiful owl. A fanfiction might follow this idea and have Harry find a young snowy owl that he bonds with immediately.
Well, he might discover a whole new world of magic in that fanfiction. Maybe it contains spells that are not in the original books. He could get inspired to try out new things or it could even lead him to new adventures within the Hogwarts library, like finding a hidden section related to the ideas in the fanfiction.