There could be a plot where Harry Potter has a strange dream about Lord Poseidon. In the dream, Poseidon tells him that he has a special mission. It turns out that there is a powerful magical artifact that was created jointly by the wizarding world and Poseidon's world long ago. But it has been lost, and if it falls into the wrong hands, it could destroy both worlds. Harry, with the help of his friends, embarks on a journey to find this artifact. Along the way, they encounter various challenges, like traps set by ancient wizards and sea - based defenses put in place by Poseidon's followers. As they get closer to the artifact, they also face the threat of a group of dark wizards who also want to get their hands on it.
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 that the Hogwarts School of Witchcraft and Wizardry is in danger of being flooded. Lord Poseidon is angry because some wizards have been misusing water - based magic. So, he decides to send a warning by starting to flood the school. Harry Potter, being the hero, has to go and negotiate with Poseidon. He discovers that there is a misunderstanding, and in the process, he learns a lot about Poseidon's world and the proper use of water magic. Along the way, he makes friends with some sea creatures who help him convince Poseidon to stop the flooding.
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.
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 crossover could be a magical duel in Hogwarts where Poseidon's powers are somehow involved. Maybe a character with Poseidon - like abilities challenges a powerful wizard from Harry Potter world.
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.
One way they could be connected in fanfiction is through a magical portal. Imagine Harry Potter accidentally stumbling upon a portal that leads him to the realm of Poseidon. There, he could learn about different kinds of magic related to the sea. Another possibility is that Poseidon takes an interest in the wizarding world of Harry Potter. Maybe he wants to use the magic there for some purpose, like protecting his underwater kingdom in a new way. It could also be that a character from the Harry Potter universe has a hidden connection to Poseidon, perhaps a distant ancestor was a demigod related to Poseidon, and this connection is explored in the fanfiction.