This is a good question, because the most important part of a paper is the thesis. In light of this, let me suggest a few possible topics.
First, how about you explore the connection between hope and strength? In other words, it takes a person of strong character to hope. To use another book as an example, The Shawshank Redemption explores this point. Red, one of the two main characters does not hope or give hope to other inmates, but Andy, the other main character, never loses hope. The whole story says that Red's position is right in view of a desperate situation, but in the end Andy is the one who was right all along. It takes great strength of character to hope. From this perspective, you can argue that the cannibalistic gangs are weak, but the boy and his father are strong. They keep the fire going.
Second, there is another possible connection. Love and hope can be another relationship. More specifically, the love between a father and son propels hope to face unspeakable things. You can also complicate this issue a bit by examining the suicide of the mother. In other words, what makes her commit suicide and the father to continue?
Finally, you can use the imagery of keeping the fire as a symbolism of hope against numerous odds.