Perhaps lower the scale. Rather than being a global organization, be a group in quarrantined Africa. Changes the mission structure from 'clear the area' to supply raids and whatnot.
Adding a base defense aspect (even if gameplay trivial) forces the player to horde supplies (your troops spend 50-100 bullets a day holding off zombies, even if you aren't playing them).
Add tension by using T.V. intermissions showing the world having (smaller) problems with a leaky quarantine, and coming closer to napalm/nuking Africa.
Since you're always locked in hell, rescuing survivors would be necessary to expand your party size. This includes researchers (Think bio-weapons and improvised weapons), fighters, medical techs, and maybe civies (civies count towards your PR rating, which could be used to get supply drops (that you'd have to go OUT to get), delay the final solution, and possibly access government tech like satelite imagery and airstrikes)
Fuel would be at a premium, so your mobility could be neatly constrained, but the late game could involve the entire continent, which means jungle, savannah, deserts, big cities, villages, wildlife, militias, ancient ruins, etc etc etc, everything you could want. Funding extra bases would be EXPENSIVE (Fuel, Ammo, defenders tied up)