Valve and Bethesda.
It'd be a first person zombie "shooter". It would have a ton of different set pieces and from them generate a random area depending on the parameters of your choosing. You could do a small, medium or large map, in a city, small town, rural countryside etc. Each map would be unique, composed of about a thousand different set pieces. Each map would have a predefined goal as set in the creation options, ranging from "get from point x to point y" to "survive for x days" or "kill x amount of zombies". There would be a heavy emphasis on scavenging, with firearms being very limited. Zombies would be numerous, and depending on your settings you could have it range from sparse to heavy population. They would be slow and require head injury to die, and they would grab the player if they got close enough. There would be a crafting component, allowing you to make improvised weapons from things you found. Food and water requirements would be toggle-able. Day/night cycles would be present. Multiplayer would be possible, with full friendly fire to allow for people to band together or fight eachother for survival. In multiplayer, items wouldn't increase, causing rations to be even more scarse.
Basically, a fully customizable zombie survival game with an emphasis on scavenging for food, weapons and medicine. Holy fuck I'd play that game.