Although I don't like the idea, I am worried that Civ might go to DLC versus expansion packs, which would very much change the complextion of expansion civs, considering DLCs are much easier to theme then full expansion packs.
For instance, in vanilla the "Native American Empire" might be in, but a DLC could split that into two or three other ones.
The same could be said for Arabia.
DLCs I could see (they would include extra goodies too, and possibly some extra leaders):
Ancient Wonders:
Sumer, Persia, Hittites
Fall of Rome:
Celts, Byzantines, Huns
The New World:
Split "Native American Empire", Incas (or Aztecs, depending on who's in), Mayans
Crossroads (trade themed one):
Mali, Vikings, Portugal