I have myself a list too, and i've based it on 3 things primarily.
First: Location and proximity to other civs, there don't seem to be great overlaps except in the middle east currently, so i doubt they will try and overlap in a more spacious area.
Second: Prestige as an empire, no-one looks kindly on the poor little Siberians
But they had a big load of space. Firaxis want a civ that's been in the wars and is famous for something.
Third (The Big One): Religion, it being a religious expansion, i've looked at civs which bring different religions to the board.
Based on this, and the fact we know 3 already these are my 6:
Sioux (or some other North-West American tribe) - this region is very lacking and we lack a rain-dance oriented religion
Austria-Hungary - Arguably the most powerful European nation in the Early Modern Era (under it's former name of Austria) and filling a void between the western civs, the ottomans and the russians
Zulu - Everyone's favourite and a sorely lacking area plus it brings a new religion this one is a dead set.
Congo - Another African area neglected another african religion and another big empire.
Carthage - Another one people have been crying out for, and the theme of this expansion isn't just religion, but satiation. North Africa only has egypt so far, carthage is pretty big etc etc It's in there.
Maratha (Southern Indian) - Hindu empire prior to British invasion, brings a religion that would only be one of many in the larger indian empire.
Or Srivijaya (Indonesia) - Buddhist empire in Indonesia before Islam arrived
Other possibilities
Chibchas (Colombia) - I doubt Firaxis can differentiate some of the civilisations down their enough to justify another civ, especially since the majority are in the northern section. If they were to add one here i would guess at the Chibchas. Plus we already got the Mayans, and central america IS south america after all.
Mughals (Northern India and Central Asia) - Big time Muslim empire in the early modern period. Built the Taj Mahal, conquered almost the whole of India at it's height. However, India already exists with the Mughal fort as its UB...
Ethiopians - Big african civ and eventually the only uncolonised portion of africa (excluding liberia) also the only christian african civilisation prior to the arrival of Europe. Interesting religiously but not unique. And it's close association to the Middle east over the rest of africa means its competing with the likes of...
Hittites and Sumer and Phoenicians and Minoans and byzantines - All fun but where are they gunna sit? The whole of the middle east is pretty much covered by what we already have. Yes, cradle of civilisation, yes thousands of years of history but there's other places... And there's only so many different names you can call the same city...
Poland - Alternative to austria i guess, never quite hits it off in European politics as much as austria but they brought in the polynesians so anything can happen
Other North american tribes - I'm not too read up on the varying tribes but i think anything could go here really, providing it's western or northern. I don't think tribes bordering Mexico or the eastern seaboard will have a say because of Maya, Aztec, USA and Iroquis.
CSA - In case firaxis find their fanbase is largely nostalgic inhabitants of the southern US. I can't see any other reason for this to be included other than the fact that firaxis is american and thus civil war is world relevant, but i think this is an incredibly unlikely scenario.
Portugal - early colonial power, but as someone has pointed out Lisbon was seen as a city state in the release photos
Bulgaria - Fairly big empire in the early middle ages in Eastern Europe, not very well known so the kind of dark horse that might spring up.
<Insert African country with history here>
Afghan or other central Asian empire - another dark horse candidate, but likely suffering from similarity to middle eastern civs and the mongolians, depending on which is chosen.
There's also a chance at various South East Asian civs like the Khmer or vietnamese joining in or even an ancient sub kingdom of China, but i think these are all far more unlikely.