One idea could be exploring the next generation of Hunters. Maybe Gon and Killua have kids who inherit their unique abilities and embark on their own adventures. They could face new threats and discover hidden secrets in the Hunter world.
First, decide on the main plot. It could be something like the ones I mentioned before. Then, create your characters. If you're using the next - generation idea, think about how the kids of the original characters would be like. Are they more rebellious or more cautious? After that, plan out the setting. Will it be in the same locations as the original series or new ones? For example, you could introduce a new continent full of dangerous and mysterious creatures.
Killua is likely to be completely devastated. He might go into a state of shock at first, not believing that Gon is really gone. Then he could become very angry, either at himself for not being able to protect Gon or at the situation that led to Gon's death.
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.