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.
For the Atlantis side, there could be Atlantean royalty, like a princess or a group of elders. There might also be Atlantean mages or warriors. And don't forget about the mer - people. They could play a role, either as allies or antagonists. Maybe there's a mermaid who falls in love with Harry and helps him understand the ways of Atlantis.
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.
One plot idea could be Percy having to deal with political intrigue in Atlantis. Since he's the king, different factions might try to influence him or even overthrow him. Another could be Percy using his powers to explore uncharted areas of Atlantis and discover ancient secrets. Maybe he finds a powerful artifact that has the potential to either save or destroy Atlantis.
I'm not sure about a specific 'Harry Potter is king of Atlantis' fanfiction. You could try searching on major fanfiction websites like FanFiction.net or Archive of Our Own. They have a vast collection and you might find something interesting there.
One main theme is the loss of innocence. The characters often face difficult situations that force them to grow up quickly and lose their naivete. Another theme could be friendship. The relationships between the characters play a significant role in the story.
The Atlantean civilization was a mysterious ancient civilization. It was said that it existed in the Atlantic Ocean more than 10,000 years ago and later sank to the bottom of the sea. Although there were descriptions of Atlantis in the works of the ancient Greek philosopher, as well as some archaeological discoveries and theoretical analysis, there was no conclusive evidence to prove the true existence of the Atlantis civilization. Some of the archeological discoveries included the discovery of strange terrain structures on the seabed near the Bahamas Islands and the discovery of huge ruins in the waters off the Caribbean Sea. However, due to the lack of sufficient evidence and archaeological excavation, the existence of the Atlantean civilization was still controversial. Even so, according to the analysis of various records, Atlantis was considered to be one of the representatives of an advanced civilization. All in all, the mystery of the Atlantean civilization had yet to be answered. More research and exploration were needed to uncover the secrets of this ancient civilization.
Atlantis is most likely a fiction. There is no conclusive archaeological evidence to prove its existence. Although many theories and speculations exist, such as the idea that it was a highly advanced civilization that sank beneath the ocean, these remain unproven. Most of the stories about Atlantis come from ancient Greek philosopher Plato's descriptions, which could be more of a moral or philosophical allegory rather than a historical account.
One such book is 'Atlantis Rising' which is full of fictional ideas about Atlantis. It explores different theories about how Atlantis might have looked, its society, and what led to its downfall. It's a good read for those interested in the Atlantis myth.