According to Steam Apps Database, who have dug through data from Steam (spotted by users on 2K Games' official forums), an expansion called One World could be coming to Civilization V.
There has been no official word from either 2K Games or developer Firaxis on the subject of expansion, but the game has only seen one major expansion so far (Gods & Kings released in June) so it wouldn't be surprising if another one was in the pipeline for 2013.