One idea could be exploring the backstories of the members of Genei Ryodan in more detail. For example, how they joined the group and what their lives were like before.
Don't be afraid to deviate from the original a bit. Since it's fanfiction, you have the freedom to explore different scenarios. For example, you could give the Genei Ryodan members new abilities or change their relationships with other characters in the Hunter x Hunter universe. Just make sure your story has a clear beginning, middle, and end, and that your characters are well - developed.
A really interesting one is 'In the Shadows of Genei Ryodan'. It takes a darker look at the Genei Ryodan, showing their more sinister side and how they operate in the underworld of the 'Hunter x Hunter' universe.
One way is through intense training. In fanfiction, a character might undergo extreme physical and mental training regimens that push their limits. For example, Gon could train in isolation with more powerful nen masters, learning new and advanced nen techniques that weren't shown in the original series. This could include more complex manipulation of his aura, like creating multiple types of nen beasts for different purposes.
The 'hunter x hunter meruem x oc fanfiction' often explores unique relationships. It might show how an original character (OC) interacts with Meruem. Maybe the story focuses on the OC influencing Meruem's view of the world, or vice versa. It could also be about their adventures together in the complex world of 'Hunter x Hunter'.
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.
I really like 'A Hunter in Beacon'. It starts with a Hunter x Hunter character somehow ending up in the RWBY universe at Beacon Academy. It has a lot of action scenes that are both exciting and true to the nature of both franchises. The author does a good job of making the interactions between the characters from different universes believable.
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.