According to the LinkedIn page of a former programmer, Firaxis may be working on an unannounced game, alongside the recent expansions for XCOM: Enemy Unknown and Civilization 5.
Christopher Haire worked on Civilization’s browser-based Facebook spin-off CivWorld, as well as an unannounced title built with the Unity engine, before he left to join casual games behemoth Zynga.Civilization Revolution is already avaliable on consoles and iOS, so it’s possible a portable XCOM title is in the works.