One of the main reasons for this thread is the ancient wonders DLC
There are many small clues pointing to the direction that Sumer and the Hittites were meant to be released as full civs, in a double civ pack
Can you elaborate on what these were? I only played the scenario once, but all I saw was that those two civs were included with existing leader screens and stills, and a new unit animation each, exactly as for the Normans and Saxons in the 1066 scenario. I don't think anyone's suggested that implies any intent of adding Normans or Saxons to Civ V (a shame actually since the Huscarl graphic was great, the Motte & Bailey too). The UAs were not UAs that would be very appropriate for full civs, either, being somewhat specific to the scenario's time period. It also looks as though Assyria's UU will not use the UU graphic from the Ancient World scenario, so they don't seem to have been "holding over" a partially-completed civ for later release.
I think if there'd been a planned civ double-pack they'd have withheld Ancient Worlds a lot longer so that development could be completed on those civs, or would have had more content for those two civs.
Another big reason is that there are still many civs I would love to see in Civ V, and obviously I'm not alone with this
While the game and the new civs are popular, probably Firaxis will want to add as many civs as possible, for further profit of course
So, there is a strong possibility that they will still keep some of the most popular civs out of Brave New World, and will add them later as DLCs
Apart from the aformentioned Sumer-Hittite civs, the Zulu, the Sioux, maybe even Portugal can get into this category (though the last is rather unlikely)
Probably there will be further DLCs after the expansion.
Also, I really hope that the remaining civs from previous civ titles will also get in eventually. Only the serious ones of course: Khmer and Mali
They are somewhat unlikely to get into the expansion, as there are some opinions against them
I'd like the Khmer back as long as they can somehow fit with the fact that Korea stole their colour scheme. They've been one of the most requested returning civs aside from Zulu and Portugal, so they probably have a good chance.
The overlap between Songhai and Mali are not bigger than the overlap between Denmark and Sweden for example, the overlap between Siam and Khmer is less then the overlap between France and Germany...
The game's Denmark and Sweden are separated by almost a thousand years. The Khmer and Siamese were very close in time and tech level, as were the Malians and Songhai. I think there's a lot of room for the Khmer at least to do something significantly different from Siam, however the game already has a lot of medieval Asian civs, which is why I expect Vietnam has a better chance than Khmer of making it into the expansion - a modern Asian civ is completely lacking.
Hungary also belongs to this category. The way Firaxis set up their Austria civ is very upsetting, medieval Hungary was one of main powers (if not the biggest power) of central and eastern Europe.
But the Austria represented in the game isn't a medieval power, it's the Austria-Hungary of the modern era, just as the Swedish civ represents a time period when Finland was part of that country.
I will also add Phoenicia to this category. Not totally the same case as for the previous 3 civs, but still close enough.
There is an overlap with Carthage of course, but it's not bigger than it was in the previous examples. Not bigger than it is between England and the USA
Phoenicia, one of the earliest (and such a successful) trading/colonizing power, home of one of the first alphabets, cannot remain absent from the civ franchise forever
It's got three city-states, which represent the Phoenician state structure well. There's no real overlap with Carthage - despite Dido's introductory greeting, the Carthaginians were not Phoenicians, except ethnically, they were a fully separate power that happened to have Phoenician ancestors.
Finally, civs probably absent from the expansion because of political reasons.
Israel, as in the ancient Israel/Hebrews, and Tibet, as in the Tibetan Empire.
These civs also have very low chance, but would also be very worthy additions, would fit into Civ V very much.
Israel would have little chance without political controversy; with it it's impossible. This was a minor nation remembered widely today only for an accident of history that resulted in the world's dominant religion being founded there, and the development of which Israelite culture had little bearing on.
So, what this means for actual (potential) DLCs:
The most popular civs, which may go as DLCs exactly because of their popularity:
Zulu
Sioux
Portugal
At least two of these are all but certain to be in BNW, the third has a realistic chance of inclusion.