I think the Military should be the most 'defensive' as really their only objective is containment, however why they dont just carpet bomb the whole city?
Zombie spawn from graveyards. Each zombie team starts with one graveyard that periodically spawns more zombies.
Miltiary units spawn from airdrops or whatever, so Military needs a Comms Centre
Survivors spawn from secret tunnels. They start with a secret tunnel.
Randomly NPC characters appear on the map that are VIPs (mayor, scientist, MIA sniper, Engineer, whatever)
Each team has a motivation to capture that person and add their skills to their team (For the military it explains why they aren't carpet bombing the place as well)
Zombies are zerg
Army is Terrans
Survivors are protoss
SURVIVORS
Real focus on individual characters. Low unit pop but no or low doubling up (i.e you can only have one of most units)
I think the survivors should be rarer, but have heaps of special skills as they know the terrain better. They may not have all the hardware of the army but they have the 'Bruce Campbell' factor of just being fucking heroes. A survivor player has to be good at micromanaging units who have lots of skills but aren't necessarily that tough.
So An individual survivor unit 'Jimmy the gas station attendant' Has a special flamethrower ability that can toast like 15 zombies easily but its on a timer. He might have other powers like 'skate or die' where he hops on his deck and skates off super fast.
Most units will be individuals or small groups (The Dirty three... a group of Mariachi's/gunslingers influenced by the film Desperado)
ARMY
Focus on modern military, track wheeled robot scouts, full spec soldiers, tactical attack choppers, laser guidance, cover, long range assaults, protocol etc.
Army has more long range firepower then the Zombies but their units are weaker. An army player values his squads and their synergy. To achieve maximum effectiveness the player has to employee complex tactics
For example a missile strike relies on several factors. The player upgrades his radar facility to enable missile strikes. Each attack takes 30 seconds to mobilize a chopper into the battle area. The chopper will then do a fly over that lasts 5-10 seconds. Army player has to enable laser targeting from one of his ground units on the target. Also if he enables two laser sights from an angle of 90 degrees or greater the damage of the attack is increased by x%
If the player fails to employee laser sighting on the target then the chopper aborts the mission and the timer is reset.
In this way the Army player uses the abilities of his ground squads to support off map and special abilities. A military player plays like a modern military outfit
Military is also bound by some rules of Engagement. Some units may no leave cover if there is too many zombies present, and if a small force of military is vastly outnumbered by scary zombies they may panic and flee to random parts of the map or cower in a corner somewhere.
Also Military cannot engage Survivors without being engaged by them first. This is set to a timer as well. Several units will have close range taser or tear gas attacks that are not bound by these rules and allow the Military player to indirectly influence the game even if the survivor does not engage
ZOMBIE
Zombies players will be rewarded for moving units in mixed packs. In this way it promotes the 'horde of undead' approach that we all want to see. Units might include meatshields, brain eaters, screamers (that increase the fear level), Heavies (good for moving debris to create road blocks, Hurlers, thrall, Nimblers (fast, weak stun units that can jump from building to building scale walls etc).
Zombies have a keen sense of smell, so unit indicators may appear outside of LOS, however these may not specifically stat what units, just that there are units in that vicinity.
Also I think Darkness, mist, fog and rain should all play a big part in the game. Zombie units, as they are primarily low tech, will be the least affected by this.
I could go on....