Has Assyria been confirmed, or is it just very strong speculation?
Out of the other 7, I think the best bets are (in decreasing order of certainty):
1. Zulu - the certainest of dead certs.
2. Portugal - again, pretty likely. I think the only reason they didn't make it into G&K is because they occupy a similar niche to the Dutch.
3. Brazil - we're due another modern Civ, and with Portugal included, the most obvious choice is Brazil - huge economy, and particularly handy for the tourism thing.
4. Sioux - another Native American Civ is obviously needed for Civil War scenario purposes; the Sioux seem to be the best candidate.
5. Indonesia/Mahapajit - again quite a good candidate based on tourism and culture. I could see the Khmer occupying this spot instead, but I think a South-East Asian Civ is quite likely.
6. Kongo/Mali - with the Scramble for Africa scenario, there's room for another African Civ. Mali have been in Civ before, but Kongo are probably more likely. I'd love to see a North African Civ like Morocco, the Moors, etc. but wouldn't bet on it.
7. Hittites/Phoenicians - I think there's room for another ancient Civ even if Assyria are confirmed.
Revised my thoughts a bit since the other day, since the addition of Brazil and the other tidbits that have trickled through:
1.
Zulu - still expect them to be in.
2.
Portugal - likewise. Brazil's inclusion if anything makes it more likely we will see Portugal as well, aside from the fact that the trade routes thing seems ideal for them.
3.
A Native American Civ - I'm now less certain that this will be the Sioux. Good cases have been made for the Comanche, and there are other possibilities such as the Navajo or Cherokee (the latter being actually involved in the Civil War to some degree). But the whole Pueblogate thing makes it all seem even more likely that they've been thinking along the right geographical lines.
4.
Indonesia/Majapahit - I don't think they are as much of a dark horse as some people have been suggesting. I'm confident a South East Asian Civ will be added and I think they are stronger candidates than Vietnam (who would be my next guess). Another trading/tourism Civ

5.
Kongo - this is where I start to get a lot less certain, but it seems like another African Civ would be a logical choice as a scenario tie-in. The Kongo Empire was pretty huge and lasted for many centuries, aside from their links with the Portuguese, so they seem like the most likely choice.
6. The 'dark horse' spot, which I am very tentatively going to award to
Canada. Brazil's inclusion has shown that modern/ex-colonial Civs are a really possibility. The clue's in the name: Brave 'New' World. I'm only about 20% sure about this one, but I think with their longer history of colonisation and the (albeit vague) possibility of integrating the Inuit culture into the Civ somehow, Canada slightly edges it over other contenders. But who knows?
Other possibilities for the 'dark horse' slot:
- Australia - see above. My next best 'modern' Civ guess.
- Belgium - definitely had a role to play in the Scramble for Africa, but other than that, I'm not sure I see them as worthy of full-Civ status compared to some of the others we're missing
- Vietnam, possibly as well as the Indonesians/Majapahit
- Mexico - not likely I'll grant you, but who knows?
- The Timurids/another Silk Road Civ - would be really neat, and fits with the trade routes, but I just don't know if they're well-known enough.
And some frequently discussed options that I think have very little chance:
- Israel - perfect as a City-State, potentially controversial (although, as I've said elsewhere, I could see a King David-era ancient Israel working - I just don't think Firaxis will go there)
- Phoenicia - somewhat subsumed by Carthage
- Sumeria - Assyria + Ur as a CS more or less rules them out
- Hittites - possibly too similar to Assyria? I'd still love to see them but don't see it happening
- Italy - I can see that there's a case for them, but their inclusion feels surplus to requirements, and the Italian City-States are fine as exactly that: City-States.