Indeed, the Maya would sell like hotcakes - but perhaps the devs would want to include it in an expansion in order to push more sales of the expansion by mere virtue of including the Maya. Or, like you said, they could just have it as a standalone DLC, giving the expansion more opportunities to include civs that fit the theme, where maybe said civs might not otherwise sell particularly well on their own (I'm sure an individual Maya DLC would sell better than an individual Colombia/Caribbean Civ DLC ever would). I suppose that could maximize their profit, if they figure enough people will still buy the expansion for every non-Maya feature it includes, and then they could sell a solid standalone DLC in addition to that. From a marketing/profit point of view, then, perhaps your theory is more likely?
Yes, you see exactly what I mean. And this goes for all of the returning civs (except Portugal or Byzantium, whichever is chosen as the eighth theme-breaking "domination" civ ala Mongolia/Ottomans). But especially for the Maya because of how well they might sell individually.
What I think we will ultimately be looking at is not whether Maya/Ethiopia/Byzantium/etc.
can be implemented, but
whether they fit expack 3's theme. If they don't, they will likely be passed over for a civ that does, and subsequently saved for future expansions, likely smaller DLC packs.
Hence why I think we need to be thinking even more holistically about which civs will be added. Not only are the devs spreading things across continents and eras and genders and old/new, but they also seem to have a clear mini-thesis for why each set of eight was chosen. It wouldn't matter, then, if Vietnam was more popular than Burma or Siam--if one fits the "theme" better than the others, that is the one we are getting.
EDIT: Upon seriously considering that, if indeed "multicultural melting pot" were the theme, then Siam would undoubtedly beat out Vietnam and Burma, being a kind of synthesis of Indian, Burmese, Chinese, and Malay influence. As much as I like Burma and others like Vietnam, if this "multicultural" theory is correct, we may end up seeing neither (barring a small DLC pack).
So here is my official guess at a "melting pot civ" pack:
Old civs:
* Siam - the ultimate Indochine melting pot. We suspiciously don't have a Bangkok CS yet. Also snags some Tai-Kadai representation no other civ can satisfy.
* Ethiopia - the ultimate Afroasiatic melting pot, with influences from Arabia, Amharic, and Judaism.
* Portugal - the Mongolia/Ottomans of the pack, doesn't have to fit any theme except be a big, go-wide civ.
New civs:
* Colombia (maybe a Caribbean civ but not likely) - perhaps the best option to represent the synthesis of Spanish and Amerindian culture by incorporating Muisca and/or Arawakan influences. Caribbean-wise we have nearly every kind of intermingling imaginable, although if we really wanted to emphasize motley there are always the Buccs.
* Timurids/Mughals - the ultimate West Asian melting pot, incorporating Turkic, Persian, Islamic, and Indian influences.
* The Native American civ. I'm going to stretch a bit here and call Navajo as impliedly intermingling with the American population, holding the largest population by tribal affiliation in the country. They have also semi-adopted many aspects of American government so there is something resembling synthesis going on even if the Navajo frame it from the perspective of the assimilated rather than the assimilators. Granted, I don't think any tribe adequately represents the idea of cultural synthesis, but the Navajo come very close and could pass.
Undecided (one new, one old):
* Morocco/Swahili - Both had at their core a very large ethnic identity which existed and thrived long before Islamic influence. Both were later assimilated into offshoot Islamic empires, developing into a blending of Islamic/African cultures. Both are represented by city-states. Both would likely have a trade bent, albeit with some differences. It's a really tough decision between these as near-equivalents, although I would likely lean toward Morocco since we already have Bantu representation in the Zulu and Yusuf is not only a clear idea for a leader bur probably one of the best options left.
* Bulgaria/Byzantium - Seems quite likely we will be getting one of these to fill out the rest of Europe. Byzantium was a kind of mix of Turkic, Greek, and Roman influence. Bulgaria was a mix of Turkic and Slavic influence. Both were regional superpowers, both already have their capital represented by a civ or CS. Again it's a battle of equivalents, although I personally feel that Bulgaria suits a "melting pot" theme more concisely and that Byzantium could easily be sold on its own in a DLC pack (if it isn't implemented as an extension of Rome instead). Bulgaria has a literal Square of Tolerance in the middle of Sofia; I don't think any civ could be more on the nose here.
In sum, most likely:
* Portugal
* Ethiopia
* Siam
* Morocco
* Colombia/Haiti/Buccs/Whatever
* Navajo (sorry Geronimo?)
* Timurids/Mughals
* Bulgaria
With possibly Morocco and Bulgaria being swapped for Swahili and Byzantium. As far as large, map-gap-filling "empires" go, this just about covers the necessities, with the only real sacrifices being a Sophie's choice between covering the Maghreb or the Swahili Coast, and foregoing Maya and Burma/Vietnam for future DLC.
(I also think that we will know roughly when the devs are done adding civs when we get at least a Copenhagen/Helsinki CS, and at least a Hanoi/Bangkok CS, two regions that are far too light on CSs).