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 Harry's struggle with his newfound dark powers. He might initially resist the pull of the dark side but gradually find that the power it offers is alluring. For example, he could use the dark magic to protect his friends in ways he couldn't with normal magic, but at the cost of his own moral compass.
One interesting prank could be to magically change the Dark Lord's robes into something ridiculous like a frilly pink dress. It would be a shock to his dignity and his followers' perception of him.
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.
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.
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 interesting plot could be the discovery of long - hidden dark magic spells within Hogwarts. Maybe a character stumbles upon a secret chamber filled with forbidden knowledge.
One interesting plot could be the exploration of the dark underbelly of the wizarding world economy in fanfiction. Maybe how certain dark artifacts are traded on the black market and how it affects the overall power balance among wizards.