I think I'd make a persistent-world zombie apocalypse co-op game or MMO, complete with:
-To-scale playable zone of one or more of the 50 United States, or the entire country, or another country, or multiple countries. Hell, with a big enough budget, we'd have the whole world playable
-Fully destructible environment, provided the means are sufficient
-Fortification system that allows players to make their own base out of whatever they find
-Vehicle customization that lets players drive any vehicle and modify it how they like
-Character customization down to shoelace color, where placement of items on the outfit affects how quickly that item can be utilized
-Crafting system that makes use of almost anything one could find
-Human needs system (hunger, thirst, sleep, waste disposal, etc.) that keeps players from just hiding, but doesn't have them running out into the city for a bottle of water every twenty minutes
-Leveling system that reflects characters' increasing proficiency with the weapons they use, along with proficiency in non-combat skills
-Combat system that features a different skill and fighting style for a large amount of different weapons, i.e. a sledgehammer isn't used the same way as a fire axe, nor are a riot baton and kitchen knife
-NPC's in their own factions that, depending on player behavior, treat players differently, as well as other factions
-Ability to toggle whether there are a finite amount of rations, munitions, NPC's, and zombies in the game world
-Resource replenishment system such as farming for food, purifying water, reloading ammunition, and childbirth for new NPC's