1. Take Assassin's Creed, or any of those sandbox games - they are pretty much the same nowadays.
2. Add a whole continent, instead of one or a couple cities.
3. Put in a dash of wastelands a la Fallout, and make sure that the zombies act like primitive humans(use weapons, drive, ability to run but no strategy).
4. Give us a car, a ridiculous amount of interesting guns, and the ability to go anywhere.
5. Add some sort of home base or many places to fortify, and make it.
6. Garnish with some challenge, but not enough to get frustrating.
7. Decorate with Co-Op, and perhaps multiplayer if done creatively enough.
8. Bake at 350 degrees and enjoy.