I think mshurn is right in identifying how John transmits the knowledge he has learnt on to his people. How and when. The short story ends with John's father counselling his son to not reveal what he has learnt about the "Gods" because his people are not ready for the truth. A really interesting story would be to narrate how and when this transmission occurs, looking at how this dystopian society advances and how they greet the truth that is presented. A very bleak story would be one where John is killed because they cannot accept his truth, and his people stick to their old ways. Humanity is then left to repeat his ignorance and forget the lesson that John has learnt through his voyage. Call me pessimistic, but I don't see humanity learning from its mistakes that easily!

You could begin a sequel with John's returning home after having been to New York. According to the story, he knows he must share what he has learned slowly so as not to overwhelm the others. Your sequel could deal with John's efforts to help his people understand what had come before. Perhaps he could choose a bright, inquisitive young boy from his people to become his student and begin to teach him the truth. This boy would become the voice and guide for his people in the future, passing on the lessons John has learned so that they will not be lost--or eventually repeated.

That's an interesting assignment.  I'll try to help with some different angles that you can take, and I am sure that other people might have some great ideas also.

1.  300 years in the future.  John's tribe has advanced quite a bit; technology has made a return, and John is a legend.  People follow the "dictates of John" in regards to culture and human behavior.  The forest people and the hill people have developed into separate nations that constantly war with one another.  A scientist in the hill nation develops the gun; they use this gun to enact force upon the forest people.  A community of "John disciples" are sad, because they recognize history repeating itself.

2.  Pretty soon after John returns, the hill people and forest people get into a war.  The hill people end up enslaving the forest people.  John leads a resistance movement, using books and history, to try to convince his elders to release the forest people.

3.  John, armed with his new knowledge, and with a realization that war caused the destruction of civilization, gathers a group of people to go on a "peace-keeping" mission to the forest people.  As they approach, they are captured and taken hostage.  Describe the after-effects, and the peace treaty that is undertaken.

Those are a few possible ideas that will hopefully get you started.  Good luck!