There's probably two different routes I would most likely take when it comes to the rules:
- Use an existing open system (D20, Microlite20, etc.) and customize it to run the game.
- Design a new system from scratch.
Every game needs a story and a big part of the story in this game is concerned with what exactly was to blame for the state of the game world today? There are actually quite a few options:
- All out nuclear war between the nations of Earth.
- A catastrophic nuclear accident that spread from one central location.
- A terrorist attack on several nuclear facilities that doomed the Earth.
- An asteroid, meteor, comet, etc. crashed into the Earth, rotated the axis, and caused death and destruction world wide.
- Something else?
The World of the Present
It is now 100 years after the event that devastated the Earth. Technology and progress has been set back dramatically. There are pockets of civilization here and there but the human population has been drastically cut down. The birth rate and survival rate is tragically lower than decades before.
Types of Characters
There should be several different types of characters to play in this game.
- (Pure Strain Humans) Some of these humans will be descendants of human civilians that made it through the catastrophe and managed to stay alive. Other humans will be descendants of military personnel that were locked away in safe bunkers; or, in homage to The Morrow Project, these military personnel could have been placed in some sort of suspended animation and awakened after the catastrophe to reestablish the nations of Earth.
- (Human Mutants) People that were mutated as a result of the catastrophe.
- (Mutant Animals) Several animal species could have been mutated and gained an intelligence level closer to that of humans.
- (Others) There could be extraterrestrial or extra dimensional species available for play also.
There should be plenty of danger and obstacles for the characters in their fight for survival.
- (Other humans) If the PCs are alive in this harsh reality, then why not other groups of humans that have banded together? Some of these could be cannibals, religious zealots, or even worse.
- (active robots) There could be places that still house old war machine robots that have come back online recently. Maybe the programming of these robots got corrupted? Maybe the robots do not recognize the players as humans if they are mutants?
- (mutant animals and plants) If the radiation affected the human PCs then it surely affected the animal life also.
- (other) Maybe the PCs could encounter life visiting Earth from other planets or dimensions?
- (environment) radiation poison zones, old military complexes, etc.