Brilliant idea! I'd buy this game if Relic or Blizzard made it today!
You'd need real small squad focus for this, like a limit of 3 to 6 squads for each side. Because if you have hundreds of survivors or soldier that'd defeat the point of horror. I see a pop cap of say:
12 men for Army
24 men for Survivors
216 for Zombies
You can form the squads yourselves, and have up to 6 men (or 18 zombies) in a squad. So you can have:
2MG + 2Snipers = Fire support squad [Army]
1 Thug + 3 Kids + 2 Hunters = Recon squad [Survivors]
15 Melee Zombies + 1 Mob leader + 2 Bone Breakers = Assualt squad [Zombies]
The point is you can choose however your squads look like, in a metagame, then these squads are your build options in the multiplayer game. Experience would be carried by individual man, and you can reform squads between games. For multiplayer, this means you can have a consistent war thing like EIR mod for CoH, or casual games where meteagame experience doesn't count.
FLAVOURING:
The Army would focus on fire power and modern tech, i.e. Elite squads fighting against the odds, Rambo style. An part tech tree would be (Note: only a small part of the tech tree, to get a feel of things), say:
(Army)
[T0] Soldiers (M16 AR)
[T1] Humvees; upgrades for M16.
[T2] M60E (The MG Rambo used); Mortars; officer.
[T3] SR25 (7.62mm Sniper rifle); upgrade mods for the Humvee.
[T4] Apache gunship / M1 Tank / 155mm Artillery depending on doctorine choice.
The Survivors would focus on stealth and superior resource gathering, moving faster using local knowledge, building barricades, using ingenious solutions, etc. Here's tech tree taster:
(Survivor)
[T0] House wives (gathers and cooks resources; uses rolling pin and kitchen knife)
[T1] kids (low profile, fast moving); Highschool students (all purpose; uses baseball bats, chair legs, short knives)
[T2] Bums (crazy melee); Thugs (good local knowledge); Doctors (heals)
[T3] Hunters (stealthy, uses rifle); Engineers
[T4] Chuck Norris (You win!)
The zombies focus on swarming, massive numbers, I think having a hundred zombies, mostly slowly advancing, but some charging, and some dogs, all coming at your 2 squads of 12 survivors with nothing but improvised weapons, is scary. Especially after you've invested a lot of time and effort into each survivor's equipement and experience. There should be a lot more frightened running away in this RTS than Resident Evil 4 ever managed. Here's the zombie taster:
(Zombies)
[T0] Infected (Max Brooks' classic zombie)
[T1] Infected animals
[T2] upgrades, more resillience, more damage, faster infection / rising rate, etc,
[T3] Specialised zombies
[T4] ?????
ECONOMIC
Army gets reinforcement points based on how little of the map they control, the less they control, the faster R points generate. But, air drop is only available every 2mins.
Survivors gets whatever they can find on the map.
Zombies has no economy, every part of the map auto generates infected units every few seconds.