Civilization 5 might be getting a new expansion titled "One World," according to a listing on the Steam Apps Database, as spotted by members of the 2K Games forums. The database lists only "Expansion - One World Content" and its executable file for PC, with no additional information. Neither 2K nor Firaxis have confirmed future DLC for the game.
The previous expansion for Civilization 5 was Gods and Kings, which launched in June. We thought that expansion brought us pretty close to the "final version" of Civilization 5, but it seems there's always room for improvement. Rome wasn't built in a day, people.