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.
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.
There's also 'Fem Naruto's Adventure in Hunter x Hunter'. This fanfic focuses on the character development of fem Naruto as she navigates the dangerous and exciting world of Hunter x Hunter. She forms unique relationships with the characters from Hunter x Hunter, like Gon and Killua, which adds a lot of depth to the story.
You can start by looking on fanfiction websites such as Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, including ones that combine Harry Potter and Hunter x Hunter. Another option is FanFiction.net. However, the quality can vary greatly on both sites, so you may need to read through some summaries and reviews to find the really good ones.
A fanfiction could focus on a character from Harry Potter who is a secret Hunter x Hunter fan. They could be constantly comparing the two worlds in their mind. Like how the Sorting Hat in Harry Potter is similar to the way Hunters are selected in Hunter x Hunter. Or how the Forbidden Forest in Harry Potter has some similarities to the Dark Continent in Hunter x Hunter.
One great fanfiction could be 'The Adventures of Killua's Little Sister'. It focuses on her learning nen abilities in a unique way, different from Killua. The story has a great plot that shows her growth and how she deals with the dangerous world of Hunter x Hunter.
I also like 'Pitou's Redemption through Gon'. In this one, Pitou is on a journey of self - discovery and redemption, and Gon plays a crucial role in that. It shows how Gon's unwavering sense of justice and his big heart could impact Pitou and help her change for the better. The story is full of emotional moments and really makes you think about the characters in a new light.
Well, there's a fanfic called 'Chimera Among the Zodiac'. In this one, the Chimera Ants have a different kind of interaction with the Zodiacs. It focuses on how their powers combine and conflict. It's a really engaging read if you like exploring the complex relationships between different elements of the 'Hunter x Hunter' universe.
There's a fanfic called 'Meruem's New Path'. In this story, Meruem survives his battle and tries to find a new purpose in the world of Hunters. It has a unique plot that keeps you hooked from start to finish. The writer does a great job of staying true to Meruem's complex personality.
One great Neferpitou fanfiction is 'The Redemption of Neferpitou'. It shows an alternate storyline where Neferpitou tries to understand the concept of humanity more deeply. Another interesting one is 'Neferpitou's Journey'. It explores Neferpitou's growth and development in a world post - Chimera Ant arc. There's also 'Neferpitou and the Lost Soul' which delves into Neferpitou's encounter with a mysterious being that challenges its very nature.
There's also 'Hunter x Hunter meets Ranma 1/2: The Adventure'. This one takes a more light - hearted approach. It has some really funny moments as the characters from both series misunderstand each other's cultures. The fanfic does a great job of exploring the comical side of a crossover.