One idea could be that Hogwarts students discover a hidden portal to Atlantis during a Care of Magical Creatures class in the Forbidden Forest. They then have to navigate the ancient Atlantean society, which has its own form of magic. Maybe they need to team up with Atlantean wizards to defeat a common dark force that threatens both their worlds.
One idea could be that the Ministry of Magic discovers a connection between Atlantis and a powerful dark magic. Harry and his friends are sent on a mission to Atlantis to stop a dark wizard from awakening an ancient evil there. They'd have to navigate the unfamiliar Atlantean magic and the strange underwater environment.
A fanfiction could involve a character from Atlantis having some sort of magic ability, similar to the wizards in Harry Potter. This character could travel to the wizarding world and learn about their magic, and in turn, teach the wizards about Atlantean technology. For example, they could show the wizards how to use some of their advanced healing devices.
One possibility is that Harry discovers a hidden magical connection between Hogwarts and Atlantis. Maybe he is magically transported there and through his unique powers from Hogwarts, he begins to assert his influence. For example, he could use his knowledge of spells to solve problems in Atlantis that the local wizards or mages couldn't. Another aspect could be that he uncovers a dark secret in Atlantis related to the Dark Arts, and he has to use his Gryffindor courage to face it and in the process, becomes a respected figure and eventually the ruler.
Maybe Harry discovers he has some hidden connection to Atlantis. He could be magically transported there and due to his powerful magic and heroic nature, he ends up becoming the king. He then has to deal with Atlantis's internal politics and protect it from external threats.
One possible plot is that Voldemort also has an interest in Atlantis. Harry, as the King of Atlantis, has to not only protect his new kingdom but also prevent Voldemort from getting his hands on Atlantean power sources. This could lead to epic battles underwater, with Harry using his Patronus and other spells against Voldemort's dark magic, all while trying to uphold the traditions and values of Atlantis.
In a fanfiction where Harry Potter is raised in Atlantis, his relationships would be quite different. He would make friends among the Atlanteans, who could have their own unique cultures and appearances. His relationship with Hogwarts would be complicated. He might not feel as attached to it as he did in the original story, or he could bring Atlantis - influenced ideas to Hogwarts, creating a new kind of magic there.
One idea could be to have the wizards from Harry Potter's world discover the Stargate Atlantis technology. Maybe they could use magic to help repair or understand some of the advanced technology in Atlantis. For example, Hermione could use her vast knowledge of spells and ancient runes to decipher the Atlantis control panels.
A great plot for a Potter Manor fanfiction could involve a mystery that surrounds the manor. For instance, strange noises at night lead Harry to uncover a long - forgotten chamber. It could also be about Harry restoring the manor to its former glory, perhaps with the help of Hermione's research on ancient magic used in the construction of such manors.
One interesting plot idea could be exploring what would happen if Harry had a different upbringing, like being raised by the Malfoys. It would completely change his view of the wizarding world and his relationships. Another idea might be a time - travel fanfic where Harry goes back to the Marauders' era and interacts with his parents and their friends. This could lead to him preventing certain tragedies or changing the future in unexpected ways. And then there's the concept of a fanfic where the roles are reversed, and Voldemort is the one trying to protect the wizarding world from a greater evil, with Harry being the main antagonist.
Well, one interesting plot could be the discovery of the connection between the wizarding world of Harry Potter and the lost city of Atlantis. Maybe the heir has some special powers related to Atlantis that help in the fight against Voldemort. For example, the heir might be able to control water in a unique way, which could be used as a powerful defense or offense in battles.