Friendship and loyalty are also common themes. The relationships between the characters in the original series are complex, and fanfics often explore these further. For example, how characters support each other in difficult times.
Sure. One great 'hunter hunter x over fanfic' could be 'The Adventure Beyond the Known' which takes the characters on a new journey full of unexpected twists. It really expands on the original world of Hunter Hunter in an interesting way.
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.