A zombie apocalypse MMO that plays out in iterations.
Here's the idea:
Build a large, detailed city with a good physics engine and a population limit. Nothing interesting happens and players are free to do whatever they want as people sign up for the game. Things continue like this for two weeks or until the population limit is reached.
Then... zombies. Heavy GM involvement here. Place a few zombies in a crowded area. See if it catches. If it doesn't, repeat a bit more aggressively. Anyone turns who gets bit. Death is permanent. When a player dies, they are given the choice of either playing as a zombie and attempting to spread the infection or starting over in a new city (with different cities running parallel on staggered schedules, so there is always somewhere one can start playing). The game keeps track of the ratio of zombie vs survivor population, and once it reaches a certain point, it becomes recognized as a full-blown zombie invasion. GMs are hands off from here, and let things progress however they will.
From this point, it's basically humans vs zombies. If the survivors can eradicate the zombie population, they win. If zombies can eradicate the human population, they win. The city has a limited amount of resources, though, and survivors will be forced to compete for them. They need food and water at the very least.
When zombies outnumber survivors by a certain ratio, the city is considered lost, and the military prepares to nuke the place. There will be frequent fly-overs dropping papers and megaphoning that anyone still alive has X amount of time to evacuate. At this stage, most survivors will be barricaded in somewhere with a hard-won store of supplies. Escape won't be easy. But if they manage it, they get to move to a different city and keep their character.
RPG System: No linear progression. It's a skill/trait point system. You get opportunities to shuffle your points around during play, but there's no leveling up bullshit.
One exception.
If you survive a zombie invasion with your character intact, you get some perks. The level of perk depends on the severity of the invasion you survived. If your city eradicated the zombies early on, you don't get much out of it. If you're the only person to make it out alive, you get something really nice. But character death is still permanent. If you die in the next invasion, you lose everything.
Additional Ideas:
1. Instead of buying a subscription like in most games, you buy a character. It's a bit of a gamble how much you'll get out of your money, of course you always have the option of continuing as a zombie if you die early on.
2. Friend/Enemy points. You can designate another player as an enemy. You get penalties on everything whenever you are near this person, except for except for actions directly targeted at the death of this person. Assigning an enemy point earns you a friend point. You can now designate a different person as a friend for bonuses to skills and such when they're nearby. Watch as rival factions naturally emerge, claim territory, and compete for resources.