Design a game

[Dalamari]

Dead Rising MMO in a city, if your character dies he/she becomes a zombie, amazing gfx and all that shit

or

EverQuest 1 remake with better cosmetics, the zones,items, and everything will stay the same, it'll just have updated graphics and all that cosmetic stuff

 

[Dekstar]

Like others said, but going more indepth:
Zombie apocalypse MMOFPS. Only the cities are slightly safe from zombies (the big ones being like a stronghold) and trading weapons/vehicles goes on but everything's pretty expensive because there's a distinct lack of fuel for vehicles, but they're good for travelling in packs (and zombies are like hella hard to kill, so if you go outside of the city alone you're pretty much destined to get owned).
Although barricades are set up around all the cities, there are zombies right outside, and they're not that stupid, so like every so often the zombies at the front would sort of let the other zombies use them as steps to get over the barricade, so they'll break into the city and start eating people and allt he players currently in the city have to defend it with iron pipes and stuff.

Also quests are economy based, and so respawning. When someone buys a weapon that's one less from the store (although, since it'd be a HUGE MMO they'd naturally have a lot of stock to begin with and respawn with), and when they begin to run out, they can ask you to go on a quest hunt for a weapons cache that respawns somewhere in the outside. You get a basic map to get to it and a free truck with fuel (the shop owners). You can create a group with anyone just wandering about, or if there's no one then you can get the AI guards to come with you. Obviously this weapons cache is something like an army base, so the zombies will be harder when you get in there and if your group dies, the truck teleports back to the shop and the quest is restarted, allowing others to try.

Rankings also occur for those who venture outside,kill zombies, and come back alive. They can go from 'Fresh Meat' to 'Zombie Master', and they all appear as badges above your head so people can see how good you are and ask you to quest with them.

Zombies are mostly AI controlled (if not all AI controlled), but it is possible to have human controlled zombies, but I think it'd be pretty boring tbh.