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.
Perhaps the Nightmare Lord is able to infiltrate Hogwarts in the fanfic through a new form of dark magic. He could be spreading nightmares among the students, which not only affects their mental state but also starts to influence the magic within the school. This would force Harry and the teachers to find a way to stop this insidious attack.
One interesting 'Harry Potter Nightmare Lord' fanfiction could be one where the Nightmare Lord is a new dark threat that Harry has to face instead of Voldemort. It might involve new magical creatures and spells.
Most likely, Harry Potter remains the main character. After all, it's a 'Harry Potter' fanfic. He would be the one to take on the challenges presented by the Nightmare Lord.
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 characters going on a magical camping trip during the vacation. They might encounter new magical creatures in the forest that are not well - known in the original books. For example, a group of friendly forest sprites that lead them to a hidden glade filled with enchanted plants.
A plot where the village is suddenly under a dark spell and the young wizards and witches from Hogwarts have to work together to break it. They'll face challenges like deciphering ancient runes and battling dark creatures. This would be a great opportunity to see different characters' skills and teamwork.