The whole game revolves around finding the last pockets of humanity, uniting them, and trying to save mankind as a whole.
It could have the mechanic of oxygen tanks and filters. To travel in the hostile enviroment, you have to wear either a gas mask, an oxygen tank or a re-breather. The gasmask slows the poison and allows short travel times in the world, while the latter are more rare, and slow you down because of their weight. ((Re-breather slowing you more, and requiring more care to keep running.))
It should handle a bit more like STALKER. Where weapons are plentiful, but you can be killed just as easily, or more so then many of your enemies.
The inventory system is somewhat like "Resident Evil", where you have limited inventory spaces, and the amount of spaces you have is based on what kind of carrying tool you have. ((A Backpack and carry more less then a trash bag, but you can only use 1handed weapons while carrying a trash bag.))
Clothing could also make a difference on your characters Armor rating, max carrying load, mobility and how people react to you. ((A nice suit is far less protective then a military grade bullet proof vest, but you cant carry as much ammo. People will be more receptive of you though, and social abilities are boosted because of how nice you look though!))
The main factions are "Civilization" ((the remaining human population, divided into sub-sects)), "The Canon" ((Rogue soldiers who survived the outbreak, and want to protect humanity, by ruling it)), "Kibbles Bandits" ((bandits who want nothing more then to get by in the hellish world, named after the working Dog Food factory they inhabit that supplies them with food)), and "The Waste" ((The Waste is made up of the ghosts of those who passed in the event. Your one of the few people who can see and interact with them.))
The Wastes present a bigger danger then any of the living factions. Most of the spiritual beings that now roam the world are completely hostile to the living, and will attack on sight, trying to take over the human's body and continue living their own lives. The only way to ward them off or kill them is through Gold weaponry and ammo, which has become exceedingly rare. Gold melee weapons are almost useless against humans, so investing large sums of goods ((bartering is the only way to get items, as all currency is seen as useless)) can get you killed if bandits decide your carrying something nice, while guns are terrible against Waste ghosts unless you literally shoot gold at them, meaning every round is the difference between living and dying.
The main goals of each faction differ greatly. Civilization wants to find a way to make humans immune to the poison. The Canon want to create gasmask factories so they can distribute them, and keep all people in line by rationing filters. Kibbles Bandits want to find and activate a secret presidential bunker, so they can live the high life in 100% clean filtered air, fresh fruit/vegetables, and all the electricity they can enjoy. While The Wastes wants to end the suffering of the ghosts that haunt it, using you as a conduit for their goals that revolves around creating and activating strange obelisks in special locations.
Each faction gives you a completely unique skill tree, revolving around different aspects of their life's. Civilization is diplomacy, making your trades worth the most, and talking your way out of tough situations. The Canon teaches you weapon skills, how to maintain them, and get the most for your limited ammo. Kibbles Bandits teaches you survival skills, making polluted food into something more healthy, and being stealthy to avoid fights you cant win. And The Wastes give you spectral powers, summoning abilities, and resistance to human weaponry. You learn these skills from half a dozen different trainers in each faction, spending fame with that faction to gain a point in the skill tree. ((But only 30 points in total out of the 50+ skills that can be learned)) And only 1 final tier skill can be learned.