Here are my guesses, by order of probability:
1- Poland is already given.
2 - Portugal IS necessary. First of all, is more important, in my oppinion, than Austria or Sweden, for instance (I really like Austria and Sweden, that's not the point); second, the international trade rotes can work perfectly with portugal. There will also be the international congress that appears when a civ contacts with all the other (portugal had this purpose in the naval expansion). Don't forgat the Africa Scenario; Portugal was the first european country ever to have a city in Africa (Ceuta, 1415), and the first european country to colonize it (only the coast areas). It will, certainly, be there. If we talk about female leaders, Maria I is a good idea. There can also be leader João II (classic) or Manuel I or even D.Afonso Henriques (Afonso I).
3 - Assyria - siege tower and yellow colours are an interesting argument to support this civilization. I would like to see Assyria instead of the Hittites. It would be good to see sumeria or Akkad (specially akkad) but they will not appear if we have assyria, for ceratin...
4- Kongo - Well, there is an Africa Scenario... Plus, Kongo was an empire that was present for more time than the Zulus and was bigger than them. The only shadow I see over Kongo is that they became vassals of portugal and this can be considered by firaxis that kongo is a minor civ (hope not)... It can also cover some modern nations that have an important role in geoeconomics like Angola.
5- Sioux - There will be needed more native american civilizations. And there is the American Scenario. That's why I think the sioux do not miss this game. You can use other native americans, but I think the sioux are the best ones.
6- Zulu - They are a mark of civilization series. I would prefer the Bantu name but ok. It would not be good to see South Africa as a Civ, in my oppinion.
7- Magyars or Hungary, as you wish - look at europeans geopolitical maps from different years and eras. What are is uncovered by civ? That's right, central-east europe! There is a big blank spot in the european map (and it is like the only spot). Firaxis, please fill this spot and then try to cover the other continents. To this region, I would either use Magyars, Ukrain or Kievan Rus'. I think the Magyars are the best ones as Ukrain had not played a big role in history and Kiev is a primordial origin of modern Russia.
8- Tibet - We need more asian civilizations and the best one is Tibet. It is an ancient civ that walked throught the times and appears in the modern world as one of the major political problems (and we have in this expansion the world congresses).
9- Brazil - there is no modern post-colonial civilization in civ V. As Brazil is the economical, cultural and scientific power of south america in the modern times, I think it will be present. Plus, there is the tourism concept (which is a bit different from beach-sun-tourism). The only problem is that Brazilian history is mixed with Portuguese history. But I think it won't be considered a problem. Let's see in this expansion (which has an awsom name by the way, I always wanted to read that book) the portuguese diaspora and the portuguese language in action!!
Supplents:
Belgium (maby change it for tibet.) - Important role in modern politics. There is a problem with belgium: it is not a coese civilization and I don't consider the nation of Belgium as a Civilization (in the right concept). But it would be very good to see it
Inuit - I think it can come in a future DLC as Polynesia. If it happens, we will have 44 civilizations (a round number. Not very important but just a curiosity).
I look forward to this Expansion!