How likely do you think it is that Killua and Gon are keeping in touch?

i can imagine it being either way, but personally i think they do keep in touch, sending each other e-mails and pictures fairly often.

however, it’s all rather superfluous, as it is bound to be between two friends that had a falling out and never talked about it. both of them probably feel the tension seeping through the lines of their letters (gon knows that killua is angry with him but refuses to talk about it with gon, and killua is an anxious mess). 

i suppose by the time they meet up again they won’t have resolved the conflict, but rather buried it under pleasantries and fake-casual conversations, which will make the whole ordeal a bit more difficult than it could have been.