Well, extrapolating a little here (this is all just heresay).
You could setup battlefield content to be worked through. that content would be fairly easy to do as static in much the way other mmo's are done today, except using an FPS based combat system instead. Ideally though, they might be a little more dynamic and let factions fight one another. I would strongly suggest they not use real countries and just invent a conflict no matter what time period was chosen.
Presumably there'd be some more RPG elements with leveling up, unlocking perks and equipment. You could still have the fast anytime action of a regular multiplayer setup, or co-op type campaigns found in other games like borderlands too. You could also have pvp battlefields where factions could vie for control of areas and resources and so forth.
Yeah it could be done, and I bet you enough people would pay for it. Activision would be very happy too; monthly sub's!