This is gonna be a Loooong list.....
Before i start typing random civs i'm just gonna say that one day it's very likely that (maybe when it's CIV V comes out

) they'll just put every single Civilization, Nation, Tribe, Country, Peoples In the world into 1 supergame. Why not wait till then?
Edit: Oh and by country i meant countries like America and Canada, not some random country like Liechstenstien.
Canada: Canada has great potential in the world in both economy and military, and allied with great countries such as america, they could be unstoppable.
Carthage: WHAT HAPPENED TO CARTHAGE!!!!! An amazing power back then, Basicly rome upgraded. Extremely suprised that they weren't included
Isreal: If the expansion has Religon in it then obviously there putting in the Kingdom of Isreal
Mayans: The great mayans should be in this expansion, The created Incredible Cities, and were the big power in mesoamerica. (oh note that on the holidays i went to Cancun for a vication, and i saw the cities for myself)
Olmecs: Not the best known civ in the world, but they were the first Civ in Mesopatamia, i'd like to see what would they cook up for the omecs if they add them.
Atlantic Tribes: Miq'mac, Mic Mac, Beothuk, Screalings, Whatever u call them, they were the first people to have contact with europeans before any other civ. (met the vikings at around 1000 AD)
Inuit/Greenlandic: Before the vikings settled greenland, these guys were there, if they were included in the game, it'll be very interesting.
Nazcas: Not much to say here but other then "hey, why not?"
Iroquois: Well Age of Empires 3 Expansion has it, and they were pretty good, if the they can do it so can 2Kgames.
Sioux: Same as above
Australian Aboriginals: Well they helped Britain very much when they came to Australia, if it wern't for them, Australia might of never been founded!
Polynesians: The Island Peoples of the Pacific Ocean, Crossing the Seas with Canoes, It's Possible that they have crossed the pacific on CANOES And landed in What is now known as Southern California, Making them the only people in the entire world to have immigrated to North America through the Pacific or the Western side. Proof is that some of the local tribes have polynesian technology. it's also sayed that they traveled to South America to. These are the native Hawaiin People to so Polynesia would be incredible to have the game.
Assyrians: Don't no much about these guys, but there there to!
Babylon: one of the civs that will just by "amazing to have"
Vikings/Scandinavia: Vikings are a MUST and having "Scandinavia" instead of vikings would be good since Scandinavia can also mean Finland and that means some techs and units from finland.
Celts: MY FAV, Celts are also a MUST. Through inhabiting the British Isles, Smack down through europe from Gaul and Iberia, All the way to russia, From russia down to Turkey and northern Germany to Northern Italy. Vast land and a great civilization, and at the peak of the empire, almost the entire contenent was theres.
Portugal: WHAT HAPPENED TO PORTUGAL! nothing more to say
Dutch: Read Above
Italy: After Rome had fallen, The roman people grew into a new empire known as ITALY. Seriously why aren't they in already?
Ottoman/Turks: Whatever u call them, they were there and a huge power.
Austria: I like austria, there a balanced nation in both economy and military, but even said that, they had good strong units, and in every game a great civ for the newcomer.
Poland: I'm Polish so i know about Poland, on constant war beetwin germany, prussia, russia, austria, hungary, and Czechoslovakia, when trouble came By the Invasion of The Mongols and the ottomans, All these Countries went straight to poland and asked poland to help. and Poland in return did and stopped these invasions.
Prussia: Small and simple, don't know if prussia counts as germany but if it doesn't, then this is added to the list.
Macedonia: The Country of Macedonia or basicly the people who first inhabited Yugoslavia. Please stop confusing this with the kingdom of Macedon which is greek.
Hungary: Not much to say but they were there in knights and castles and stuff
Czechoslovakia: Read above
Yugoslavia: The kingdom of Yugoslavia, Who can foreget them?
Albania: not exactly a power but they were in those little wars with yugoslavia and Macedonia.
Venice: I don't remember much bout these guys but i think they envaded parts of italy, yugoslavia, greece(including the island of crete) when they were finnaly tooken over by italy.
Minoan: The people of Crete
Malta: The smallest nation i'll add, but they do have a very nice history.
Nubia: People south of Egypt
Etheopia: Prople of Etheopia
Great Zimbabwe: Zimbabwe
Zulu: i know NOTHING about these guys other then they are tribes in south africa.
Mali: i don't remember but these were the biggest Civ south of the sahara
Ghana: Ghana
Congo: The people of congo
Vietnam: Rounds up Asia
Malaysia: Well Malaysia is always cool.
Indoneysia: Islands in SouthEastern Asia with People and a Civ.
I geuss i'm finished but these are the top 5:
1. Celts
2. Carthage
3. Poland
4. Vikings
5. Polynesia