Yes, transformation can absolutely be a theme. It often adds intrigue and a sense of progress to a novel. For example, a character could transform from being shy and withdrawn to confident and outgoing, or a dystopian world could transform into a utopia through the efforts of the characters.
I don't think so. There haven't been any official announcements or indications that the HxH manga will continue. But you never know, maybe there'll be a surprise in the future.