The epilogue of this book is meant mostly to suggest directions for further research. Diamond wants to indicate what sorts of questions historians should pursue.
Importantly, Diamond also has something to say about how historians should try to do their work. He thinks that history should become more like a science. Historians should, he says, use natural experiments (like the one about the Polynesians that he describes in Chapter 2) to uncover truths about human history. He believes that historians should start to do this so that they will be able to make more definitive statements about history.
So, the overall purpose of this epilogue is to suggest that historians should use these natural experiments to make history more like a science.