Perhaps in this fanfiction, Harry is born with a tail due to some ancient magic that was dormant in his family line. This tail could be a sign of a special power he has yet to discover. He might have to go on a journey to find out the origin and purpose of his tail, with his friends Ron and Hermione helping him along the way. They could explore hidden chambers in Hogwarts or ancient wizarding libraries to uncover the mystery.
One possible plot could be that a character from Fairy Tail gets accidentally transported to the Harry Potter universe but is neglected by the magical community there. They then have to find their own way to get back while also discovering new magical abilities and making unexpected friends.
The plot could involve a dark magic spell gone wrong. Harry gets cursed with cat ears and a tail. He then has to find a way to reverse it while dealing with the strange new sensations and the reactions of his friends. Some might find it hilarious, while others might be worried about him.
Maybe it could be due to a misfired spell in Hogwarts. Some student might accidentally cast a spell on Harry, and instead of the intended effect, it makes him grow a tail. It would be a really strange and comical situation for him.
A plot could involve a symbiote attaching itself to Harry. This symbiote might have its own motives. It could be from another dimension where magic and symbiotes are intertwined. The symbiote could start influencing Harry's actions, making him more aggressive in his fight against Voldemort. But at the same time, his friends are suspicious of the changes in him. As the story progresses, Harry has to learn to control the symbiote and use its power for good while also dealing with the internal struggle of not losing himself to it.
Maybe Harry is at a Quidditch match. He's flying high in the air on his broomstick, but suddenly he feels the urge to pee. He can't just fly down to the loo in the middle of the game. His teammates notice his distracted state and wonder what's wrong with him. It could lead to some interesting interactions among the Gryffindor team members as they try to figure out what's up with Harry while still playing the game.
Perhaps the plot could be that the child of Harry Potter gets sent back in time accidentally. There, they meet a young Harry and his friends. The child has to be very careful not to disrupt the past too much while also trying to find a way back to their own time. This would be filled with lots of tricky situations, like when they almost reveal future events or when they have to avoid being seen by their own young parents.
In a Harry Potter has a son fanfiction, the plot could involve the son inheriting his father's magical abilities and facing new threats at Hogwarts. Maybe he discovers a hidden chamber that holds a powerful and dangerous magic. He might also have to deal with the pressure of being Harry Potter's son, with everyone having high expectations of him.
The plot could involve Harry finding the scythe in a hidden chamber at Hogwarts. Maybe it was left there by an ancient wizard, and the scythe has the power to cut through dark magic. Harry then has to learn how to use it to fight Voldemort in a new and unexpected way.
In a fanfiction plot, Harry could be born with telekinesis but it was suppressed by the magic that protected him as a baby. When he enters Hogwarts, the magic of the school starts to awaken this power within him. His friends are both amazed and a bit scared at first. As he hones his telekinesis, he uses it to uncover secrets in the castle, like hidden passages that were sealed off long ago. But this also attracts the attention of dark forces, and he has to use his power to defend Hogwarts and the wizarding world from a new threat that is related to an old, forgotten magic.
One possible plot could be that the maid is a magical being in disguise. She could be secretly helping Harry Potter with his tasks at Hogwarts, like cleaning up after his potion experiments in a much quicker and magical way than normal. Maybe she has some hidden knowledge about the Dark Arts that she imparts to Harry in a very subtle way, which helps him in his fight against Voldemort.