Metro 2033 developer 4A Games is set to unveil plans for the future -- as in next week, not post apocalypse. The game's official Twitter account stated yesterday: "Like a fine single malt, the DLC is almost done ... we hope to be able to take the wraps off it this week."
We don't know what items will be added to the atmospheric foreign-person shooter, only what won't be included: nothing multiplayer ("not possible") and "it won't be new SP [single-player] narrative content, 'missing levels' or anything like that." In fact, the DLC only began production back in March. We'll be sure to let you know more as soon as we hear it, seemingly later this week.