@PhoenicianGold- I think those are pretty fair points! I do think that it's certainly possible more design space could open up with expansion mechanics- for example, if more intricate politics and governance were introduced, Byzantium could have abilities blending that system and religion in some way! At least if I were designing it, heh. Portugal I could still possibly see as its own civilization, too, though I think I'm with you on Babylon at this point. Do you think a Babylonian leader leading Sumeria would work?
I think new mechanical design space has more potential for some civs than others. I would say that design and power creep is a major factor in determining whether a civ "fits" with expansion concepts.
When looking at the major ideas left to be introduced, I think Byzantium, for as unnecessary as I personally find it, could find things to work with. If we introduce more city state politics like vasselage, protectorates, or colonies, Byzantium is a solid candidate to illustrate those mechanics. You could argue that Rome would be equally deserving of these features, but on the whole the distinction stands up where Rome tended to claim and lose regions by force, whereas Byzantium's influence and legacy in surrounding regions was largely cultural.
But in that same vein, Portugal doesn't make sense as a beneficiary of these mechanics, because everything Portugal did regarding protectorates or colonies, Spain arguably did better. While we can understand why Phoenicia and the Ottomans are on a higher complexity level than the base game, making Portugal "complicated" just isn't a very resonant idea. Part of that is sharing quite a lot of historical parallels with Spain, and part of that is lacking anything particularly different or uniquely Portuguese to work with. See Scotland and the Netherlands for contrast.
When the thing you are most known for is colonizing the world with religion and trade, and then you were acquired and incorporated into the Spanish empire for some time, and *then* Firaxis chooses to make Spain fit precisely that mold, what else do you have, Portugal? And to bring things back around for emphasis, what about Portugal justifies it to be one of the most mechanically complicated civs in the game, above and beyond Spain?
So regarding a Babylonian leader for Sumeria, I've gone back and forth on the idea in the past. There was a time when I thought Hammurabi or Sargon or Shamamurat could slide into Sumeria. But unlike other blobby civs, I just don't think it could work. Because these leaders would be standing next to
Gilgamesh, who is not only Mesopotamia incarnate but a total ubermensch. They all come close, but at the end of the day Gilgamesh is smarter and more poetic than Hammurabi, stronger and more manly than Sargon, and sexier and more sensitive than Shamamurat. By design Sumeria leaves no space for anyone else, Gilgabro is the only man you will ever need.
These examples are why I think the Kingmaker may be onto something with "renamed" "clone" civs. Because there exists this other spectrum of civs, ranging from the Mughals, (early) Byzantium to Kievan Rus' to Huns to Akkadia to Portugal, that players want
on principle but don't seem to acknowledge or care how little potential they represent for gameplay. And I'm guilty of this to an extent as well, since I really want some Mughal/Kievan/Hun representation but acknowledge that they could and probably should just mechanically be slight variants on India/Russia/Scythia and could comfortably be alternate leaders. It would not surprise me if a lot of remaining "staples" are ultimately introduced as "clones" of base game civs. So that players have the leader animations, the city lists, the retermed "uniques," maybe new music -- but we don't need to suffer the awkwardness that would result from giving these "staples" weird, niche, nonresonant playstyles.