So this is just throwing some more speculative oil on the fire. I have no idea what I'm talking about, I've played, like, 6 games so I don't even know if I've seen all the city states in the game yet. But I have noticed that most of the CS are usually the same, most of the ones I've seen have popped up in at least 2 or 3 games. And it's actually relevant, somewhat, in Civ VI, in V it was just change the name and it's a new CS, in VI they have unique abilities.
Sydney was not featured as a CS which I did notice. On the flip side Toronto is. So it would make sense they would put Australia in the game, and leave out Canada.
There is no Cusco in the game, nor any CS that I recognize as being from Peru. On the flip side La Venta is in the game. Now La Venta is technically Olmec, not Maya but based on this I would certainly expect the Inca to be one of the relatively sooner additions to the game, probably before the Maya.
Another CS that is notably lacking is Vienna. Prague is featured in the game, but not Vienna. So I would definitely expect Austria to be one of the sooner additions to the game.
There's also no Karakorum or anything resembling a Mongol CS, I mean at no point did Mongol society at all resemble a CS, but this might point to the Mongols being featured soon.
There's AFAIK no Tehran or Persepolis or anything like that, could mean Persia will be added, not necessarily though as it wasn't really a city state.
The real big one that IS featured in the game? Seoul. With a very powerful, unique ability, too. So I do not expect Korea to be added to the game before the expansion. Again, baseless speculation here but I would expect they will re-do the CSes at the expansion, so the ones left out are likely to be released before while the ones included would not until after.
Amsterdam and Lisbon are in that category too, they aren't as unique but they are featured. So I do not expect a Dutch or Portuguese civilization before the first expansion, I would really hope not anyway, there is already too much Europe going on especially if they add Austria. Carthage is also featured, so, again, it'll probably be an expansion civ.
Also not speculation but I really want to see Ragusa and Florence as CSes, come to think of it Florence might be in but I'm pretty sure Ragusa isn't.