Recently Xcom: Enemy Within was spotted in Steam's database, something that naturally spurred speculation about a possible sequel to Xcom: Enemy Unknown. Firaxis, developers of Xcom: Enemy Unknown, are set to reveal their next game at Gamescom this month, so perhaps there may be something to this sighting.
Xcom: Enemy Unknown was recently released on iOS, and The Bureau: Xcom Declassified - a spin-off from 2K Marin, is set for release very soon.