XCOM website has the official announcement of XCOM 2, a sequel to XCOM: Enemy Unknown in development at Firaxis for release on Windows PCs in November, with versions also in the works at Feral Interactive for Linux and OS X. IGN says this is indeed the cause of 2K's recent teasing, offering a reveal trailer described like so: "Watch the debut trailer above for a glimpse into a future where the aliens are in control of the Earth, and XCOM has gone underground to fight to overthrow their Advent government. This new guerrilla force will face more powerful enemies in unpredictable combat scenarios as they fight to turn the tables on a technologically superior enemy." They promise their exclusive access to the developers will result in the revelation of "<trump voice>huge</trump voice> details" over the coming month. In the meantime, here's the thrust of the story from the announcement:

In XCOM 2, the roles have been reversed, and XCOM is now the invading force. They are hampered by limited resources and must constantly evade the alien threat in their new mobile headquarters. Players must use a combination of firepower and stealth-like tactics to help XCOM recruit soldiers and build a resistance network, while attempting to expose the evil alien agenda and save humanity. XCOM 2 will introduce gameplay features such as procedurally-generated levels, which will make each experience unique to the player, as well as offer a much deeper level of modding support. Additionally, XCOM 2 will offer a variety of new content including five updated soldier classes, increased soldier customization, more alien and enemy types, evolved tactical combat and more.