When the game starts, you're alone in a bunker. You'd cross off another day on the calendar and start to check your gear. You prepare to leave for the nearest town; you strap on your gas mask and pull up your hood. The green light on your oxygen gauge tells you that you'll be able to survive in the airless environment outside. The world was brought to a complete standstill when the air started dissipating.
Most didn't know what was happening before it was too late. It wasn't how humanity had figured it would end. All in all, it was a very silent, quick affair without the big explosions or the screaming masses fleeing. In fact, no one could have imagined a quieter apocalypse. People simply fell where they stood or sat at the time. There wasn't time to evacuate, and where would have people gone anyway? Those who survived were the people who had planned ahead, who had planned for the impossible. Their reward was solitude on a lifeless, airless planet. But at least there's plenty of time to think and reflect.
The end happened so quickly that the destruction was limited. You have entire cities open to exploration and treasure hunting. Only... you have to watch your air gauge. The air gauge itself is on your suit/clothes and not on your UI, so you have to look down to see it. If you forget it, you might end up fucked unless you can somehow find air.
You move around empty schoolyards, apartment buildings, and skyscrapers. The lack of air preserved the billions of bodies worldwide relatively longer than they otherwise would have been, and now the empty planet is a mausoleum of dried up human husks littering streets and buildings.
You'd have to scavenge for stuff to improve your gear so you could stay out longer. Through exploring, you might find other safe bunkers and you'll be able to move more freely. Then there would be enemies. What would they be? Something that doesn't require air, which could stalk you and make you feel watched. A less open threat that might attack when you least suspect it. And would there be others?
Maybe you'd have to find them first. Would you even go armed? No air would mean no sound, so all you'd have was the clicking of the gas mask and maybe whatever contemporary band you managed to put on that banged up, scavenged iPod you managed to find in the pocket of a body near the airport terminal. You'd have to consider getting fuel for the generator in your safehouse, as well.
Your only contact with other people would be someone who you talk to frequently over what's left of the internet or some old radio-type thing. You've known them for years, but have never met them in person. They're your only company in an otherwise dead world, and they're holed up somewhere else, exactly like you.
One day, they stop signing in/going on the air and you decide to find out what happened to them. You only have a faint idea where their bunker is and you know you wouldn't have enough air to get there in one stretch. That's where the game really begins. Do you make an effort to find them or what happened to them, or do you go on with your life, salvaging, exploring, and listening to your own breathing as you make your way through the still graveyard of humanity?