There are two ideas, both biggish ones.
1. No one knew for sure when or where the outbreak started. It encompassed town after town, with the government keeping it secret for as long as it could. It was a zombie virus, transmitted only by bites or direct contact. The US governments response was to keep it a secret, restrict information, but after a while, with the zombies moving on big cities, the time for discretion was over. I would be playing a marine lieutenant, in charge of a platoon, sent to a particular street of LA on the outskirts of the city to hold back the horde, along with the other platoons in my company, with other platoons placed on specific streets to hold them back. Your character would live on that street. They could be ex military, or current, back from a deployment.
2. Earth was the centre of a Federation, advanced, and trading with other races. A new race, powerful as well, declared war on us, turning some of our allies against us, and promising others our technology if they refused to honor defence pacts that they had signed. The war raged for two years, until the race and its allies were almost at Earth. We had planned for it though, setting up an ark deep under the former Cheyanne mountain military complex. It had cryogenic chambers for 15,000 of Earths best and brightest, vehicles, enough ships for all of us, food stores, armories, vast databanks with our latest developments, powered by what we called Dark energy, very powerful, and couldnt be detected by any scanners. Dark Energy was what the race had been after.
The race and its allies wiped out every human it could, razed our cities, and refused to honor its promises to its 'allies', sending them packing and taking over control of Earth and what technology it could find. They were after the ark, but it was well hidden and shielded. A thousand years later,a virus, released by something the race found on Earth, or that developed on its own, hit the race hard, decimating its numbers, and a hundred years later, the race was extinct, a victim of its own greed.
Humans had escaped the races grasp, settling on hidden worlds, or with allies who sheltered them. With the Race wiped out, humans could return to Earth, to rebuild, and find the ark for themselves. After 1100 years, only my cryopod remains for the descendants of those who managed to escape to find.
If anyone wants to play out either idea with me, let me know.