Being a AAA game and all, XCOM: Enemy Unknown seems primed for some downloadable content. A few hidden Steam achievements might give a clearer idea of what to expect from them. Be warned, though, these might include a few minor spoilers if you're particularly spoiler-allergic.
A user on the 2K Forums (via PC Gamer) uncovered the achievement descriptions. Some achievements are for completing missions like "Confounding Light," "Deluge," "Friends in Low Places," "Gangplank," "Portent," and "Furies." Those all sound like mission names, though some could be packaged together as part of a DLC mission pack.
The descriptions give a few more details. Confounding Light involves a turn timer, Deluge involves activating valves, and Friends in Low Places implies a unique squad member. Our money is on an alien team member of some kind, hopefully wearing a cowboy hat and learning important lessons about life and love on this crazy planet we call Earth.