One interesting plot could be a character from an unexpected house rising to lead all the houses in a time of great peril at Hogwarts. For example, a Hufflepuff might show extraordinary leadership skills during a dark wizard attack.
There could be a plot where the sorting hat's magic goes awry and students start to develop abilities associated with different houses than their own. This would lead to interesting character developments as they struggle to adapt to these newfound traits and the reactions of their original house mates.
One interesting plot could be the Head of Houses having to deal with a new, mysterious threat that only they are aware of at first. They might have to work together secretly across house rivalries to solve the mystery.
Maybe in the fanfiction, as Lord Potter, Harry discovers ancient Potter family magic. This newfound magic could give him an edge in his fight against Voldemort. For example, he might find a spell that can protect Hogwarts in a more powerful way than the existing protective enchantments.
One interesting plot could be a new adventure where Harry discovers a hidden power related to lightning that was unknown in the original series. This power might help him face a new dark wizard threat.
One interesting plot could be a new adventure where Harry discovers a hidden chamber in Azkaban that holds a powerful magical artifact. This artifact might be related to an ancient wizarding family and its secrets. Another plot could involve the Dementors having a different origin than what is known in the original series, and Harry has to find a way to stop them from spreading a new kind of dark magic.
A possible plot could be a crossover battle. A powerful dark wizard from the Harry Potter world teams up with some sea monsters from Poseidon's domain to attack both the wizarding world and the underwater world. Harry Potter and Poseidon then have to form an alliance to stop this threat. They would have to combine their different powers and strategies. For instance, Harry could use his spells to hold off the dark wizard while Poseidon uses his power over the sea to deal with the sea monsters.
One interesting plot could be Harry Potter exploring his connection to the Peverell family heritage in more depth. Maybe he discovers long - lost magic spells or artifacts related to the Peverells.
One idea could be a character who has to balance the different cultures and traditions of the multiple houses they are lord of. For example, Gryffindor's courage, Ravenclaw's intelligence, Hufflepuff's loyalty, and Slytherin's cunning all come with their own sets of values and expectations.
Well, one interesting plot could be the internal struggle of the son. He might be torn between the dark heritage from the Dark Lord and the good influence of Harry Potter's world. Maybe he has to hide his true identity while trying to figure out his own moral compass.
One interesting plot could be that the rich lord offers to sponsor Harry Potter's education at Hogwarts secretly. This way, Harry has access to all kinds of rare magical items and advanced spells. Another plot could be that the rich lord is actually a long - lost relative of Harry's and reveals some hidden family secrets that change Harry's perception of his own identity. Or perhaps the rich lord has a powerful magical artifact that Voldemort desires, and Harry has to protect the lord and the artifact from Voldemort's clutches.
One interesting plot could be the Nightmare Lord creating new and terrifying dark creatures that Harry and his friends have to face. These creatures might have unique powers that challenge the heroes in ways they haven't experienced before.