Start simple just to get the ball rolling :
- "Living" faction are the normal PR ones, so nothing to do here. (better to concentrate on zombies and frankly lets you profit from all the other factions for variety "Zombie world Tour!"

)
- Create a zombie faction in the game files
- Create a zombie kit (civi with only hand to hand available) but with either more health or lots of armor (except the head)
- Create basic zombie texture + throw in a cow moo as the sound file ^^
Rules:
- Use Insurgent mode as a base, aim is for survivors to last the longest not to win
- Zombies have low respawn
Use Al basrah as the test map as you have a good mix of terrain features.
Get this up on a locked test server and start eating brains.
Afterward bit by bit add these (for example):
- Zombie kits variety
- Barricade creation system (then make it use "scavenge" points)
- Implement a new zombie spawn system to get that "horde" feeling at times
- Zombie "Vehicules" (if you want more exotic shaped zombies)
- Textures & sounds, but low priority compared to having solid game features.