Okay, let me start.
1. CANADA
Some Canadians never cease to try and promote their country as though it where a civilization. If colonies spawned by their masters had certain historical names, then the first I would choose would be Canada for England. But Canada is NOT truly that important historically, culturally, militarily, and (forgive me Canadians) Diplomatically. I personally would rather live in Canada than the US, but I know America is WAY more important than Canada ever has been and most likely will be.
2. Gran Colombia
Gran Colombia was a state that lasted only 12 years of bickering, fighting, and eventual dissolution that separated it into 3 dictatorships. It BARELY won a war against Peru, my home country, which I'll readily say was a complete internal mess that managed to survive together until the time of cultural cohesion and national identity. Gran Colombia was basically a failed Bolivar plan. Bolivar actually shot his own dreams in the foot because he separated Bolivia from Peru in personal self-recognition. Guess where Bolivia got it's name from. This one basically covers anyone advocating a Bolivarian State.
3. Brazil
Brazil is where all the hot women come from
That should be enough, in my opinion. But Brazil as an important influence in the world didn't occur until the latter part of the 80s, and even then it was just meetings with India, trying to establish common ground for the third world. Regardless, Brazil is huge, and it's culture WILL spread throughout South America. Uruguayans all learn Portuguese, if I remember right. It's kind of like English and the rest of the world. Some Argentines and Paraguayans learn Portuguese as well, knowing that the future of South America rests in Brazil. Northern South America(essentially colored South America) has been sheltered by mountains and distance from Brazil's culture, but that won't last for long. My dad, a Peruvian patriot(somewhat loosely using the word), advocates for Peru to become a province of Brazil. Brazil has THAT nice a future ahead of itself. Maybe in 2050 once the dust settles about the future of the world, but not now, not yet.
Those are the Big 3 that immediately come to my mind. The following are just random guesses on my part.
4. Kongo, Benin, and other African Empires not in previous games
Fireaxis just doesn't seem to like going out of it's comfort zone of well-known civs. Benin was a VERY important empire to African history, and Portugal acknowledged it and it's king as equals, not as a colonial master. The Benin had beautiful architecture, art, and a capital city of power. Eventually, the British got a hissy fit and razed Benin City to the ground, and built a much more impoverished version, destroying all the art, culture, and architecture that made it one of the most beautiful signs of African Architecture ever. However, they'd make everybody go 'Huh?????', so Fireaxis will most likely not do it. The Khmer where a nice suprise, but somewhat known. They could suprise us with Benin.
5. Siamese, Indonesian-based, Vietnamese, and other Southeast Asian Empires not called the Khmer or possibly Burmese-based Empires
I'm honestly wondering-Did any of these countries actually build, conquer, or influence any significant amounts of territory, people, etc.? The only thing the Siamese and Vietnamese empires did was carve up a Khmer empire in decline, like the Arabs and Bulgars to the Byzantines. Except the Arabs and Bulgars where more important. Indonesia-based Empires where mostly trading/pirate empires, and did not really leave a significant mark anywhere except make the straights of Malacca not safe and have to be patrolled first by the Chinese, then Dutch, then British.
6. Central Asian Empires
These Empires are actually INCREDIBLY important, but not one actually established an empire for more than a century at best. The Huns folded like paper once Attila died. Other later nomads, such as the Central Asians under the Mongols, where part of a larger empire, Meanwhile, some controlled major trade routes and chose to never migrate(Uzbeks). The question is, does Turks satisfy 'Central Asians' to Fireaxis? Biggest question, IMO.
7. Migrating Empires in Europe and the Middle East
This sort of goes with the above, significant powers that crumbled and disappeared with a minor trace. The Goths are the most prominent example, followed by the Franks. The Goth's most established settlement was near Western Ukraine, but they came from Scandinavia. They conquered much of Coastal Russia, Poland, Western Ukraine, North Africa, Eastern Spain, and harassed Europe (mostly Rome) to no end. However, not one of these Goths managed to form a stable empire. Eventually, they too crumbled to later mass emigrations from other Germanics, which brings me to the Franks. They too migrated, into France. They, however, won, beat up more, beat up some more, and eventually controlled pieces of Spain, all of France, Germany, and Northern Italy. This is NOT the HRE. The question is, why was Charlemagne the leader of the HRE. Anyway, I doubt we will see either the Holy Roman Empire or the Franks. Fireaxis will most likely want to cover it this way:
Where we want to sell(USA) -> Very important(China) -> Geographically important(Mongols) -> Historically important(Spain) -> Tradition(Aztec/Zulu)
Then after all of these spots are taken up, we will have or openings for the quite important civs. These will most likely also try to cover geography. Hence my argument of no more European civs as an immediate release apart from the standard variety. I'm going to summarize more european civs as so: Being more euro-centric is not something they would want to do. Anyway, discuss your opinions regarding which civs WILL NOT appear despite being popular.
1. CANADA
Some Canadians never cease to try and promote their country as though it where a civilization. If colonies spawned by their masters had certain historical names, then the first I would choose would be Canada for England. But Canada is NOT truly that important historically, culturally, militarily, and (forgive me Canadians) Diplomatically. I personally would rather live in Canada than the US, but I know America is WAY more important than Canada ever has been and most likely will be.
2. Gran Colombia
Gran Colombia was a state that lasted only 12 years of bickering, fighting, and eventual dissolution that separated it into 3 dictatorships. It BARELY won a war against Peru, my home country, which I'll readily say was a complete internal mess that managed to survive together until the time of cultural cohesion and national identity. Gran Colombia was basically a failed Bolivar plan. Bolivar actually shot his own dreams in the foot because he separated Bolivia from Peru in personal self-recognition. Guess where Bolivia got it's name from. This one basically covers anyone advocating a Bolivarian State.
3. Brazil
Brazil is where all the hot women come from

Those are the Big 3 that immediately come to my mind. The following are just random guesses on my part.
4. Kongo, Benin, and other African Empires not in previous games
Fireaxis just doesn't seem to like going out of it's comfort zone of well-known civs. Benin was a VERY important empire to African history, and Portugal acknowledged it and it's king as equals, not as a colonial master. The Benin had beautiful architecture, art, and a capital city of power. Eventually, the British got a hissy fit and razed Benin City to the ground, and built a much more impoverished version, destroying all the art, culture, and architecture that made it one of the most beautiful signs of African Architecture ever. However, they'd make everybody go 'Huh?????', so Fireaxis will most likely not do it. The Khmer where a nice suprise, but somewhat known. They could suprise us with Benin.
5. Siamese, Indonesian-based, Vietnamese, and other Southeast Asian Empires not called the Khmer or possibly Burmese-based Empires
I'm honestly wondering-Did any of these countries actually build, conquer, or influence any significant amounts of territory, people, etc.? The only thing the Siamese and Vietnamese empires did was carve up a Khmer empire in decline, like the Arabs and Bulgars to the Byzantines. Except the Arabs and Bulgars where more important. Indonesia-based Empires where mostly trading/pirate empires, and did not really leave a significant mark anywhere except make the straights of Malacca not safe and have to be patrolled first by the Chinese, then Dutch, then British.
6. Central Asian Empires
These Empires are actually INCREDIBLY important, but not one actually established an empire for more than a century at best. The Huns folded like paper once Attila died. Other later nomads, such as the Central Asians under the Mongols, where part of a larger empire, Meanwhile, some controlled major trade routes and chose to never migrate(Uzbeks). The question is, does Turks satisfy 'Central Asians' to Fireaxis? Biggest question, IMO.
7. Migrating Empires in Europe and the Middle East
This sort of goes with the above, significant powers that crumbled and disappeared with a minor trace. The Goths are the most prominent example, followed by the Franks. The Goth's most established settlement was near Western Ukraine, but they came from Scandinavia. They conquered much of Coastal Russia, Poland, Western Ukraine, North Africa, Eastern Spain, and harassed Europe (mostly Rome) to no end. However, not one of these Goths managed to form a stable empire. Eventually, they too crumbled to later mass emigrations from other Germanics, which brings me to the Franks. They too migrated, into France. They, however, won, beat up more, beat up some more, and eventually controlled pieces of Spain, all of France, Germany, and Northern Italy. This is NOT the HRE. The question is, why was Charlemagne the leader of the HRE. Anyway, I doubt we will see either the Holy Roman Empire or the Franks. Fireaxis will most likely want to cover it this way:
Where we want to sell(USA) -> Very important(China) -> Geographically important(Mongols) -> Historically important(Spain) -> Tradition(Aztec/Zulu)
Then after all of these spots are taken up, we will have or openings for the quite important civs. These will most likely also try to cover geography. Hence my argument of no more European civs as an immediate release apart from the standard variety. I'm going to summarize more european civs as so: Being more euro-centric is not something they would want to do. Anyway, discuss your opinions regarding which civs WILL NOT appear despite being popular.