Don't be so sure. With the exorbitant production values in games these days we're seeing more and more rely on expansions to provide content which was previously expected of titles only five years ago. Think of Half Life 2, Warhammer, Dawn of War, Starcraft, Disciples III. I wouldn't be too surprised if we got a marine-only campaign in the initial release, and expansions for the other two races; disappointed maybe, but not surprised.