There seems to be a noticeable pattern, at least in the first two DLCs: two highly marketable civs (Great Britain and Carthage in CofW, Assyria and Silla in RtR) + two civs that might not sell on their own (Nepal and Bulgaria in CofW, Qajar and Dai Viet in RtR). Silla may not be immediately recognizable to casual players or those who aren’t history nerds, but it's well-known enough for the Korean audience.
Personally, I believe the upcoming DLCs will follow the same pattern, and there are still plenty of highly marketable civs left to come: Ottomans, Edo Japan, Aztecs, Byzantines, Brazil, Netherlands, Poland, a Celtic civ, a Viking civ, Ethiopia, Portugal, Haudenosaunee, Zulu, Babylon, Sweden, more Korean versions, and possibly even more Chinese versions.
There are also civs that have never appeared before but have a decent demand and could serve as selling baits: Goths, Burma, Ireland, Ashanti, and maybe Nazca and Minoans.