First, understand the rules and lore of both universes thoroughly. For Hunter x Hunter, know about nen, the different types of hunters, etc. For Harry Potter, understand magic, Hogwarts houses, etc. Then, come up with an interesting premise that combines elements from both. For example, a character from Hunter x Hunter discovers they have magic like in Harry Potter. Keep the characters' personalities true to their original selves and let the plot develop organically as the two worlds collide.
I really liked a fanfic where the Slytherin descendant is a girl who discovers her heritage and tries to change the perception of Slytherin in Hogwarts. It shows how she uses her Slytherin traits like cunning and resourcefulness in positive ways. She forms alliances with students from all houses and they go on adventures together. It's a really heartwarming story.
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.
Yes, often there are. For example, the main character as a descendant of Merlin might have unique magical powers. They could be able to perform spells that are long forgotten or have an innate understanding of ancient magic. This can lead to all sorts of adventures, like exploring hidden chambers in Hogwarts that were sealed by Merlin himself and uncovering secrets that could change the wizarding world.
Yes, there are some out there. You can try searching on fanfiction - specific websites like FanFiction.net or Archive of Our Own. Some authors might be inspired to create crossovers or unique takes on the idea of Harry Potter being related to the Witch King, often exploring new magical concepts and character developments.
There's also 'Hunters at Hogwarts'. In this fanfiction, the Hunters end up at Hogwarts for some reason, perhaps a strange nen - based portal. The cultural clash between the two groups of characters is really fun to read about, and the author weaves in elements from both series seamlessly, making for a really engaging read.
Perhaps an ancient evil that Gryffindor once sealed away is starting to break free. This entity sees the descendant as a threat because they have the potential to reseal it. So, it tries to corrupt the descendant or stop them in any way possible.
One interesting plot could be that the descendant discovers a hidden chamber in Hogwarts that was sealed by Gryffindor himself. Inside, there are ancient spells and secrets that could change the wizarding world. The descendant has to battle dark forces who also want to get their hands on these secrets.
Often, there are other characters from the original Harry Potter series as well. For example, some fanfictions might include Hermione as a friend or mentor to the descendant. She can use her intelligence to help the main character understand their powers better. And of course, there could be some original characters created specifically for the fanfiction, like a mysterious wizard who also has a connection to Merlin's past.