Okay so some dark horse candidates:
A while back I took at look at the top ten pages of most popular mods and noted that quite a few of the additions/changes seemed to align with popular mods on steam. As far as new civs, a Greece Split (P. 1), the Norse (P. 2), Sumeria (P. 3), and Kongo (P. 7), and as far as switching up leaders, Qin Shi Huang (P. 1), Peter (P. 3), India/Maurya Split (P. 4), Victoria (P. 4), Teddy (P. 8), Cleopatra (P. 8), Philip II (P. 8), and an Ayyubid Caliph (P. 10). That makes up 2/3 of the base roster with new civs or leaders. Brazil and Aztecs kept their leaders from V, so really only France, Germany, Rome, and Japan took left turns from expectations. And Scythia was just a curveball attempt at a kurgan culture.
Then, if we look at the DLC, only two new civs weren't in the top 10 most popular pages. Canada (P. 1, P. 9 with Laurier), Australia (P. 3), Scotland (P. 3, P. 4 with Robert), Mali (P. 4), Angevin Empire (kind of blobbing Francia (P. 4) and Normandy (P. 6); Eleanor is somewhere past page 10), Hungary (P. 6), Maori (P. 7), Khmer (P. 7), Phoenicia (P. 9), Cree (P. 9), Poland (everywhere), Maurya (P. 4). Nubia falls somewhere beneath P. 10 but the design very closely resembles the VI design. And Macedon seems mostly like it was included for the sake of Alexander who was already in V. Point being, the DLC selections also seemed somewhat crowdsourced from Civ V mods.
Quick disclaimer: these are the numbers as of over a year ago which might have shifted a bit, and only sorted by "most popular". I remember doing a rough check against "most subscribed"; they vary here and there but for the most part you see a lot of the same civs popping up. I also just checked and Gran Colombia is currently on page 3 of most popular.
Looking at other popular civs on steam mods, and refining it a bit from last year, let me note some things which we might see:
Top three page civs:
* The Goths. By far the most popular civ, and the most subscribed outside of Axis powers.
* Vietnam. Also one of the most popular civs, generally page 1 or 2. On the same level as Canada before it was included.
* The Inuit/Greenland. Also more popular than nearly any other civ, again page 1 or 2, and later on page 6. I think the Maori get a pass because they were a deblobbed Polynesia, but I think if the Maori didn't have the precedent of being part of a Civ V civ, no one would be seriously considering them. I think a Northern equivalent might actually happen purely because of how popular the Inuit are.
* Bulgaria and Oman. Bulgaria is quite popular, P. 3 (I just checked, P. 8 on subscribed). Oman is also popular, P. 5 (and P. 3 on subscribed). These are about as popular as Scotland and Australia. As prior new inclusions show, this doesn't rule out anything below page three, but I'd say these are the two biggest competitors remaining in the top three pages.
Sheer numerosity:
* Alternate leaders for China, Russia, and Germany. These have so many alternate leaders among the top ten pages that I'm pretty sure the devs are considering it. Seems like we will get our wish with Kublai Khan. Might eventually get something like Arminius/Bismark or Olga/Lenin.
* Finland and Afghanistan. Finland is on page one for both popularity and subscription, and appears multiple times. Afghanistan also appears multiple times in the top ten pages of both lists. While both of these feel quite comfortable as city-states, they are insanely popular, Finland especially. They could happen based purely on demand (the Saami are also within the top five pages so they might be used as a more interesting Finnish stand-in).
* Something in the Caribbean. There are a lot of Cuba and Pirates civs in the top ten pages. While I am perfectly satisfied with just the Taino city-state, the Taino don't really cover Cuba nor Buccaneers. I could see at least one more city-state, or possibly like Finland/Afghanistan a full civ if the devs think there is demand. Though maybe the Taino were intended to fill this spot.
* The Sioux, Papal States, and Jerusalem show up more than once in the top ten pages. Two of these are city-states and I don't see the Sioux happening in light of the Cree. But we might see a Soiuan city-state like Blood Run.
I admit, this is a weird list. Definitely doesn't cover my wishlist like the Timurids (P. 6) or Burma (P. 10). But if the developers have been looking toward mods as indicators of popularity (which I think they have also been doing with city-states), this is what the numbers seem to say.