One idea could be exploring an uncharted area of the Dark Continent that has unique and deadly creatures not yet seen in the main series. Another could be a story about a group of hunters from different families teaming up specifically for a Dark Continent expedition, facing internal conflicts as well as external threats.
Well, 'Hunter's Expedition to the Dark Continent' is a fanfic worth checking out. It's not as long as some others, but it packs a punch. It shows the different perspectives of the Hunters going to the Dark Continent. The descriptions of the strange creatures and environments are vivid, making it feel like you're right there with the characters in the story.
First, do some research on what is already known about the Dark Continent in the Hunter x Hunter series. This will give you a base to build your story on. Then, think of a unique concept or character that you want to introduce. It could be a new type of creature or a hunter with a special ability.
Well, the atmosphere. Since the Dark Continent is supposed to be this extremely dangerous and mysterious place, a fanfic that can really capture that feeling stands out. It should have a sense of foreboding, like at any moment, something really bad could happen to the characters. And the pacing is important too. If it can keep the story moving forward, not too slow or too rushed, while still exploring the wonders and terrors of the Dark Continent.
The Hunter x Hunter manga is unfortunately not complete. The author has faced various issues that have led to intermittent breaks and an unresolved storyline.