I might be misunderstood
@Abaxial. I was not judging if a civilization is relevant or powerful, but how probable is a civilization to come back. If the developpers could add all of them and made them all interesting, I will be happy. But the number are limited. Here, I am just giving an opinion on which civilization are likely to come back.
I even made a
sheet in the process!
Civilization VI makes it clear that they would focus more on a civilization that gives them idea for new mechanics and ideas to exploit than trying to represent all the "relevant" civilization of the world.
Austria appeared one time in Civilization V, and Maria Theresa also lead the Germans in Civilization II (it was different time). So, I am give the civilization a score of 1.2 out of 6 for representation in the franchise. The thing is: the Hungarian kind of stole the idea. In Civilization V, Austria was about friendly annexing city-state with Hussar as unique unit, meanwhile Hungary is about city-state leverage with Huszar as unique unit. As you said, Austria-Hungary was a major European power. But I guess they went for Hungary this time instead of Austria. If they bring back the Austrian, they will probably focus on other ideas.
I kind of have two "problems" with Ireland (this is going to be controversial).
The first is the over-representation of english-speaking countries like England, USA, Australia, Scotland and Canada. Adding Ireland makes the number at 6. That is why if a new Celtic civilization is going to appear, I think the Gallic or Brittany (french side) are more likely. But I will be also okay with Ireland.
The second is the Armagh city-state. They did change some city-state in the past, but none of them add a unique tile improvement. Well the Monastery is rather neutral, so it will be easier than finding a replacement for La Venta. As I said, I am not against Ireland, I am just trying to figure out what it is the most likely.
Iroquois was more represented in the franchise than Austria (2 out of 6!). This is a piece of information not relevant, but hey: that's a fact! Anyway, I agree the Iroquois is very unlikely to come back.