Another plotline could involve Harry's relationship with the other houses. Since he is now seen as the heir of Slytherin, the Hufflepuffs might be cautious around him, the Ravenclaws might be intrigued by the historical implications, and the Slytherins themselves could be divided between those who welcome him and those who resent his sudden appearance as their heir.
Perhaps there could be a plot where Harry discovers ancient Slytherin magic that only the heir can access. This could lead to him having to make difficult choices between using this power for good or being corrupted by it. He may also have to deal with the Slytherin house's expectations of him as their heir, which are quite different from what he has known in Gryffindor.
One interesting plotline could be the exploration of the true identity of the Heir of Slytherin. Maybe it's not who everyone suspects. For example, it could be a character who has been hiding their Slytherin heritage all along.
One interesting plotline could be Percy having to deal with the Slytherin values while still holding onto his own heroic nature from the Percy Jackson series. Maybe he has to face prejudice from some Hogwarts students who are suspicious of a 'half - blood' being the heir.
One possible plotline could be Harry using his new position to reform Slytherin house. He might change the way Slytherins are perceived in Hogwarts, promoting unity among all the houses.
One great 'Harry Potter Slytherin heir' fanfiction is 'The Heir of Slytherin's Redemption'. It shows the Slytherin heir in a new light, exploring their journey to break free from the dark expectations. Another one could be 'Slytherin's Hidden Heir'. It delves into the mystery of a previously unknown heir with a lot of plot twists. There's also 'The True Slytherin Heir' which focuses on how the heir discovers their true identity and purpose within the wizarding world.
Perhaps a plot where Harry, as the heir of the founders, has to unite the different houses in Hogwarts in a way that hasn't been done before. He could use the unique knowledge and powers associated with being the heir to bridge the gaps between Gryffindor, Slytherin, Hufflepuff, and Ravenclaw. For example, he might find a way to resolve long - standing rivalries and bring the students together to face a common threat.
One interesting plotline could be Harry discovering ancient magic secrets related to his status as heir. Maybe he finds a hidden chamber in Hogwarts that only the heir can access, filled with powerful spells and artifacts.
One interesting plotline could be Harry exploring his new role as an heir and the powers or responsibilities that come with it. Maybe he discovers hidden family secrets or a new form of magic.
One interesting plotline could be Harry discovering ancient Gryffindor secrets. For example, he might find a hidden chamber within the Gryffindor common room that holds powerful magic only accessible to the true heir. Another could be his interactions with the Gryffindor ghosts in a more profound way, where they guide him to understand his new role as the heir.
There are various plot ideas. Some fanfictions might explore the idea that the heir of Slytherin is not a single individual but a group of people who have been secretly working to change the reputation of Slytherin house. They could be working against dark wizards within the house who are trying to revive the old ways of pure - blood elitism. Or perhaps the heir has a connection to an ancient magic that is tied to Hogwarts itself, and in order to save the school from a new danger, they must unlock this magic. It could also be a story where the heir is in a love triangle with a Gryffindor and a Hufflepuff, adding a romantic element to the plot while still dealing with the challenges of being the heir of Slytherin.
One interesting plotline could be the exploration of the Slytherin Prince's backstory. Maybe he has a hidden family legacy or a past encounter that shaped his personality. Another could be his relationships within Slytherin house, like rivalries or unexpected friendships. Also, his interactions with Harry Potter in a different light than in the original books, perhaps as an ally in some situations rather than an enemy.