In certain 'Harry Potter Heir of Atlantis' fanfictions, the plot could revolve around the political intrigue within the wizarding world due to the emergence of the Heir of Atlantis. There could be different factions trying to either use the heir for their own gains or trying to eliminate the heir because they fear the power that Atlantis represents. This would create a complex web of relationships and power struggles similar to what we see in the original series.
Some fanfictions might have a plot where the Heir of Atlantis is on a quest to find ancient artifacts from Atlantis that are hidden within the Hogwarts castle. These artifacts could hold the key to saving the wizarding world from a new dark threat. Along the way, the heir has to solve a series of puzzles and overcome obstacles, just like in the original Harry Potter adventures.
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.