Another theme is redemption. Fanfic authors like to explore the idea of Akutagawa making up for his past wrongs. He might be given a chance to save someone or change an event that he regrets in his past. It gives a new dimension to his character.
One great BSD Akutagawa crossover fanfiction is 'Akutagawa in Another World' which explores what would happen if Akutagawa was transported to a different universe. It has an interesting plot where he has to adapt to new powers and a new society.
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.
Most BSD light novels often have complex characters. Since they are based on literary figures, they bring in the depth and complexity of those original personas. For example, a character might have the same traits as a famous writer but with a unique twist for the BSD universe.
Gon in fanfiction can be very single - minded in his pursuit of goals, just like in the original. He might be a bit more naive at times, which can lead to interesting plot twists. Killua is often portrayed as having a complex emotional side. He struggles with his past as an assassin and his loyalty to Gon. His cool exterior may crack in some fanfictions, showing his true feelings.