This is a very interesting hypothetical question to consider. Clearly both Oryx and Crake and The Year of the Flood cover similar themes regarding the dystopian end of the world thanks to man's indifference to and abuse of nature. Both novels are set in the same world that has been destroyed by man, and feature characters desperately trying to come to terms with what has happened and struggling to hack out some form of existence. Yet, what is interesting from my perspective is the way that really Atwood uses these themes of the destruction of the environment as a backdrop to explore the psycholgical development of characters such as Jimmy in Oryx and Crake. Any subsequent sequel that Atwood would create to form an environmental trilogy I imagine would view this frightening dystopian world from yet another perspective and reflect upon humanity and what has brought us to this stage in our existence.