So we're expecting two civs between Brazil and Poland and two between Portugal and Zulu. They've announced the civs in a way that leaves practically the whole alphabet open, but it does make a few things clearer. These are all civs that people have floated in various threads (I weeded out some of what I thought was really unlikely, but obviously left a lot of left-field possibilities in there):
Ruled out: Afghanistan, Australia, Argentina, Armenia, Asanteman, Anasazi, Apache, Belgium, Benin, Berbers, Bornu, Bangla, Bhutan, and Bohemia.
Possible: Any two of Burma (Pagan), Chachapoya, Cherokee, Chile, Colombia (Gran Colombia), Comanche, Cree, Crete (Minoans), Cuba, Hittites, Hungary, Indonesia, Inuit (Eskimo), Italy, Khazaria, Khmer, Kush (Nubia), Kushan, Laos, Mapuche, Merina, Mexico, Morocco (Moors), Navajo, Nazca, Nepal, or Pakistan and any two of Romania, Scythians, Seminole, Sioux, Timurids, Toltec, Uzbeks, Venice, or Vietnam.
Indonesia, Italy, and Morocco are clearly the favorites from the first group, and the Sioux, Venice, and Vietnam from the second. Indonesia could theoretically show up under a different name (Srivajaya, Majapahit); Italy and Venice are presumably mutually exclusive. Evidence is very strong for either Italy or Venice being in, and pretty strong for Morocco (definitely wouldn't say "99%" though). Because Firaxis had planned to introduce the Pueblo, a Native American civ seems likely, but that's not certain either. Somebody pointed out that there's a Silk Road achievement that only names existing civs, which might imply that Khazaria, Kushan, etc. aren't going to make it.
It's interesting that the second group is so much smaller. Obviously there's nothing very scientific about the way we've come up with potential civs, but it seems to me like that's a pretty thorough list and some of them are very improbable. Scythia, the Timurids, the Toltecs, and the Uzbeks are all very close to existing civs (the Huns, Persia, the Aztecs, and Persia again, respectively). That just leaves the three favorites, plus Romania and the Seminoles, both of which seem extremely unlikely to me. By comparison, the group of civs between Brazil and Poland seems wide open.
Any chance the Sioux don't make the cut? I kind of doubt we'll get Venice and Vietnam, and what else could go there? They've been in the Civ series before, and they do have a pretty good leader and some interesting cultural bits and bobs, but I have to admit I'd be a little disappointed by a Sioux civ. Pueblo definitely would've been way cooler (and, actually, there's still a remote chance that the Pueblo are in—and they'd fall into that second group).