Perhaps there's a plot where a magical creature that has the appearance of a snake is disguised as a lion - like being. It could be used as a sort of spy within the Gryffindor common room. This would lead to all sorts of chaos as it tries to gather information for an unknown Slytherin - related plot.
In some fanfictions, a Slytherin student might don the 'clothing' of Gryffindor behavior to infiltrate their group. They could be trying to steal the Gryffindor sword or find out the secrets of the Gryffindor house. This would involve the Slytherin having to constantly maintain the facade of being a brave and chivalrous Gryffindor while secretly plotting against them.
Another option could be a new character. A transfer student, perhaps, who is really a Slytherin at heart but is placed in Gryffindor by mistake. This character then has to keep up the pretense of being a Gryffindor while dealing with their true Slytherin nature.
Perhaps the snake is a transformed wizard who was cursed. This would lead to a plot where Harry has to find a way to break the curse. He might have to go on a quest to find rare ingredients for a potion. Along the way, he and the snake face various challenges, like dark wizards trying to stop them, and they form a strong friendship through their shared experiences.
One possible plotline could be the discovery of a new type of deadly snake in the Forbidden Forest that has some mysterious connection to dark magic. Maybe the snake is guarding a long - lost artifact.
Perhaps in the fanfic, Harry is turned into a snake for a period of time. As a snake, he experiences the wizarding world from a different perspective. He has to learn how to survive as a snake while still trying to fulfill his role in the fight against Voldemort. He might encounter other snakes that are either friendly or hostile, and he has to use his wits to deal with them.
Well, a plotline might involve the snake familiar getting into trouble. For example, it could be mistaken for a dark creature by the Ministry of Magic. Harry then has to prove that his snake is not evil. Or perhaps the snake helps Harry discover a hidden part of Hogwarts. Since snakes are often associated with secrets, it could lead him to a secret chamber or passage that no one else knows about. And this could be a key to solving a mystery or finding a powerful magical item.
In a 'Harry Potter King of Lions' fanfiction, a great plot could be that the lions are guardians of a powerful magic that can only be accessed by someone with a pure heart like Harry. The King of Lions tasks Harry with a series of trials. These trials are not only physical but also test his moral compass. For example, he has to choose between saving a group of innocent magical creatures or obtaining a magic item that could give him an edge in the fight against Voldemort. Through this journey, Harry grows in his understanding of magic and his own self - worth.
One interesting plotline is when Lily is given the chance to change the future. In some fanfictions, she has the power to go back in time and make different choices, like whether to marry James or not. Another is when she discovers hidden magic within herself that no one else knew about. This often leads to her having to face new challenges and enemies.
Some fanfictions focus on the snake as a misunderstood creature. In this type of story, the snake is constantly misjudged by the wizarding world. Harry, being the empathetic character he is, befriends the snake and discovers that it has a heart of gold. They then go on adventures together, with the snake using its unique abilities like slithering into small spaces to retrieve magical items for Harry.
One interesting plotline could be about the Founders' initial meetings and how they decided to build Hogwarts. Maybe they had different ideas at first but then came to a consensus on the school's values and layout.
Well, perhaps in this fanfiction, the Phoenix plays a role in revealing hidden secrets about Harry's family. It could lead Harry to discover an ancient magic that was long forgotten by the wizarding world, which was somehow related to his family. This could then set him on a new quest, different from the one in the original books, and change the course of the story completely.