Zombie mmo. A full out blown one, with a dedicated single player mode set in the pre-apocoliptic stage, where the infection is still spreading and shopping centres are being looted, fights are breaking out and riots are only making things worse. It would act as a tutorial for the actual mmo as well, teaching combat, improvised weapon development, and other cases to help settlement development.
Oh yeah, the mmo's gonna have settlements built from scratch by the players. No capital cities, no lore important npcs-everythings determined by the players themselfs. Groups would have their guild/gang/whatever build their own town, and have people with different backstorys doing different jobs (Gathers for food, tacticians for combat and settlement defense, scientists for upgrading others usefulness, doctors for helping injured members, builders for barricades and basic houses ect ect) and the only help they would get is if they built their settlement by say a farm for food, or a block of houses for occupation. These would be limited however, and town wars to steal locations or resources would be common. These settlements would have to use efficent management at the beginning, with npcs only appearing to help once the players show competence (Builders leveled their skill enough to repair a partally collapsed house? they now automatically repair slightly damaged barricades even when offline). Global zombie spawn rates would depend on the current amount of people who have joined the game, and the relative sizes of the settlements to constantly keep a challenge and prevent the whole game being civiliazed.
There would be no quests either- your expected to help in any way possible to help develop your settlement, such as going into the nearby zombie infested lab to gather notes on how a bite spreads the disease (these kinda places would run like instances in wow, and never be civiliationable), which upon collecting would allow doctors in your settlement to begin development on a suppessor for your explorers, which would prevent zombie bites from turning you when active.
Oh yeah, and if you die, you die for reals. If you make a new character afterwards, you would keep any items you had on you (storyline explanition would be that you found them on previous characters corpse), instantly be re-accepted into your old settlement ect ect, but with your previous professions reset to basics.
Theres more I have in the back of my head, but that would go wayyyyyyyyy to far in.