Looks like there's a followup to XCOM: Enemy Unknown, as the title "XCOM: Enemy Within" has appeared both on the Steam database and the South Korean classification board. The latter lists PC, Xbox 360 and PlayStation 3 versions.
It remains to be seen if this is DLC, a standalone expansion or even an actual sequel to XCOM: Enemy Unknown.
XCOM: Enemy Within spotted on Steam database, Korean ratings board
01 August 2013 | By JonahFalcon