The most interesting interpretations I've come across really zero in on the end of the anime and that painful separation. A lot of stories treat it as the starting gun for two completely different, parallel growth journeys. Gon's arc is often about the aftermath—the guilt, the physical and emotional recovery after his self-destruction, and learning to wield that intense passion without it consuming him. Killua's is, predictably but fascinatingly, about breaking free. Not just from his family, but from the self-imposed cage of being 'Gon's protector.' The best fics don't just reunite them for fluffy comfort; they rebuild the friendship on totally new, more mature foundations. I read one where Gon, years later, had to learn to ask for permission before touching Killua, and Killua had to learn to say 'no' without fearing he'd lose everything. It was such a simple, brutal reflection of how deep those old wounds went.
Sometimes the growth is less about healing trauma and more about exploring the paths the manga didn't take. I've seen Killua leaning into his assassin heritage but on his own terms, becoming a force for a twisted kind of justice, while Gon completely loses his Nen and has to find a new purpose as just a normal, incredibly determined kid. The dynamic shift when one of them is suddenly the vulnerable one opens up so many possibilities. The growth feels earned because the fic has to dismantle the old power balance first.