Phenomenal strategy reboot XCOM: Enemy Unknown may be getting several new missions via DLC if some suggestive Steam achievements are to be believed.
Inquisitive PC players posted an image of said strange Steam achievements on the 2K Forums, noting that their descriptions aren't visible on your profile page. Many of them seem to be awards for completing specific missions - such as Confounding Light, Deluge, Furies, Gangplank and Portent - that don't appear in the original game, strongly hinting at a future expansion or series of DLC packs.
2K and Firaxis are keeping quiet on the matter at present, which is probably for the best since XCOM: Enemy Unknown is already massive enough to devour any and all free time you have. I described it as "a sensational game and a truly exquisite reboot" in our 9/10 review.