It's a bunch of clues. This image file was found in the packed assets after the latest patch, and it has a picture of the three missing wonders of the seven ancient wonders, as well as Sejong. Further, the text files make references to a DLC pack 6, a hittite, assyrian, and sumerian civ, and there are lines in the gameplay XML for there to be Mausoleum, Statue of Zeus, and an Artemis Temple.
Also, long time ago a mysterious german poster claiming inside knowledge (I know, sounds weird, huh?) Claimed the next DLCs would be, in order: Spain and Inca, Polynesia, the Danes, Korea, and the Wonders of the Ancient World. People laughed at this, but then the next packs were Spain and Inca, then Polynesia (which had seemed unlikely) then Danes (which was definitely a million-to-one guess if it were, that it would be the "Danes" when they were Vikings or Scandinavia in the past Civ games). So we think the German knew what was up, in that case the next two DLCs are Korea and then Wonders of the Ancient World.
Finally, the Korean Civ5 affiliate has a contest "guess the leader of Korea" running through the first week of August. 30 unnamed achievements have been released onto Steam, which always happens right before a DLC announcment but 30 is more than one DLC usually requires.
So this is not "confirmed" but a lot of things make it seem extremely obvious.