The theme of destiny often pops up. If Harry is a royal heir, it's seen as part of his greater destiny. There's also the theme of secrecy and revelation. The fanfictions usually start with Harry not knowing about his royal heritage, so there's a build - up to the revelation and then dealing with the consequences of that secret being out. It adds a lot of drama and intrigue to the story.
One common theme is power. Since Harry is a royal heir, there's often exploration of the power that comes with that status, both in the wizarding world and potentially in the muggle royal world if there are connections. Another theme is identity. Harry has to grapple with this new identity as a royal heir on top of being 'the boy who lived'.
One common theme is the discovery of one's heritage. The heir often has to figure out their connection to a great wizard like Harry Potter or one of the founders. Another theme is the struggle with power. Since they are an heir, they may have access to great power which they need to learn to control. And there's also the theme of facing challenges from those who oppose the heir's new - found status.
One common theme is self - discovery. Harry, as the heir of Emrys, often has to figure out what this new identity means for him and how it changes his place in the wizarding world.
One common theme is identity struggle. Harry has to grapple with being associated with Slytherin, a house known for its darker tendencies, while being a Gryffindor. Another theme is the exploration of Slytherin's past and its secrets. Writers often dig into the history of the house, its founders, and the magic associated with it. There's also the theme of friendship and acceptance, as Harry's friends have to come to terms with his new identity.
One common theme is the clash and cooperation between magic and Muggle technology. The Royal Navy represents the height of Muggle nautical technology, and when combined with Harry Potter's magic, there are a lot of interesting interactions. For example, using magic to enhance the speed or defenses of the Navy ships.
One common theme is power struggle. Since the character is a royal descendant, there are often others who either want to use them or get rid of them to gain power. Another theme is identity discovery. The character usually has to figure out their royal heritage and what it means for them in the wizarding world.
Power and responsibility is a big one. Since the character has a connection to Merlin, they have to deal with the power that comes with it and the responsibility to use it well. Also, the exploration of ancient magic and how it fits into the existing Harry Potter magic world is quite common. For instance, they might explore how Merlin's magic is different from what is currently taught at Hogwarts. There could also be themes related to prophecy and destiny, as the character's new identity as the Heir of Merlin may be tied to some greater purpose in the wizarding world.
One common theme is the discovery of heritage. Harry finding out he has this connection to King Arthur and then trying to understand what it means for him.
One common theme is the exploration of the Black family heritage. Writers often delve into the secrets of the Black family tree and how the heir is connected to it. Another theme is redemption. Since the Black family has a dark past in some aspects, the heir might be on a journey to redeem the family name. Also, power struggles within the wizarding world related to the heir's position are quite common.
Another common theme is the struggle against dark forces. In the Harry Potter universe, there's always a threat of dark magic. A Royal Phoenix Animagus might be seen as a symbol of hope or a powerful weapon against the dark. Fanfictions may show the character using their unique abilities to fight against Voldemort or other dark wizards. They might have to overcome their own fears and insecurities to face these powerful enemies, which adds to the character development and the overall tension of the story.
Power is a common theme. Since Grindelwald was all about power and so is Harry in his own way, the exploration of power - how it's used, abused, and controlled often comes up.