I think the following four are shoe-ins, based on evidence we've seen and past Civ history:
1. Poland
2. Assyria
3. Portugal
4. Zulu
As for the other five... I dunno. I suppose we could look at it by region/era:
Ancient Mesopotamia:
I don't think we'll see any more ancient Mesopotamian civs. If they're adding Assyria, that'll be enough. I'm sure they'll quit with that.
Africa:
With the Scramble for Africa scenario and the historical inclusion of Zulu in Civ games I felt comfortable enough to add them to the top 4. I think Mali are out, as others have mentioned they are basically replaced by the Songhai. Ethiopia, Zulu, Songhai, Egypt & Arabs probably wouldn't be enough. Only three of those would be in sub Saharan Africa.
So I like the suggestion for the Kongo. Especially since they were around during the Scramble for Africa.
Americas:
I think Brazil is long overdue. In all the years of civ we've never had a modern South American civ. Seems to be quite an oversight. With the rise of the BRIC nations,
I think Brazil's chances are pretty good.
Mexico or Argentina have a small chance, but I seriously doubt it. I don't really see the Sioux being included. People keep talking about a tie in with the Civil War scenario, but as others have mentioned, it's focused more in the east. There has never been a game of Civ (except Civ:Col of course) where there is more than one North American native civ. I don't see them starting now.
Asia:
People keep suggesting Khmer, but with Siam in the game I doubt they'll add in Khmer as well. Just like I doubt they'll have both the Mali & Songhai. Maybe Khmer will be back in Civ6 or 7, but I don't think it'll happen here.
I very much like the suggestion of Majapahit or Indonesia. Another increasingly populous/important nation right now who should get their due, they occupy an otherwise empty part of the map, and have had some history.
I'm not sure who else they would add. The Mughuls maybe? Generally they've been included under India, but who knows. I think it'd be pretty neat to put them in, but I wouldn't bet on it. Israel perhaps? I dunno.
Europe:
Ah, good ol' crowded, ethnocentric Europe. Poland is in. Some have suggested Belgium... ugh. Under any other circumstance there is no way Belgium would get in, but since we've got a World Congress feature and a Scramble for Africa scenario,
Belgium is looking like a real contender. No offense to Belgians, but the idea of adding Belgium makes me a bit ill...
Ill because including Belgium makes it much less likely they'll add Hungary. With as crowded as Europe already is I can't see them adding FOUR of the nine new civs from Europe. Poland, Portugal and probably one more. And damn, Hungary needs a shot. The country been around for more than 1,000 years, has made a real impact on Europe, and has been neglected by Civ for much too long. Poland has been in much the same situation, another great civ that just never got its due. So with Poland in could that make Hungary a possibility? I'd sure like to see it, but... probably not.
Another possibility as a dark horse candidate would be Venice. As others mentioned, Venice and trade routes couldn't possibly go together any better. They'd definitely have a trade related power. I think they'd have a stronger case for themselves than modern Italy.
So my guesses for the last five are:
1. Kongo
2. Brazil
3. Majapahit/Indonesia
4. Belgium
5. A darkhorse like: Mexico/Argentina/Mughuls/Hungary(please!)/Venice/Italy/Israel