No continent is going to get all the new tags, or even half of them, in a hypothetical final pass. So suggestions with 8 american civs or 5 European ones are prety unlikely.
After the original batch of DLC, pretty much every single expansion can be broken down as follow:
2 European Civilization : Gaul/Portugal (NFP), Sweden/Hungary (GS), Netherland/Scotland (RF)
1 Eurasian Civilization (as in, stradling the border between Europe and Asia): Byzantium (NFP), Ottoman (GS), Georgia (RF)
2 Asia-Pacific Civilizations: Vietnam/Babylon (NFP), Phoenicia/Maori (GS), Mongolian/Korean (RF)
2 American Civilizations (1 North, 1 South): Gran Colombia/Maya (NFP), Canada/Inca (GS), Cree/Mapuche (RF)
1 African (Subsaharan) Civilization: Ethiopia (NFP), Mali (GS), Zulu (RF)
I'd assume a similar pattern for an hypothetical final expansion, with the only change I'll make being replacing the Eurasian Civilization with an Eurafrican one. So 2 European, one Eurafrican, 2 Asia-Pacific, 2 American and 1 Subsaharan African. With that breakdown in mind, my list would be:
Italy, Goths, Berbers (including Berber Al-Andalus), Philistine OR Hittites, Siam, Iroquois, Guarani, Swahili.
Italy is pretty much the only choice left for a Marquee European civilization to get players attention. The Balkan and Central-Eastern European options that are left, except Austria, just lack the attraction power to work here. One of the two has to be in, and because I have another idea for Maria Theresa, Italy gets the nod.
With a fairly mid-to-late-game centered civilization, I want my second European spot to be earlier in the game (much as with Portugal and Gaul). And let's face it: the people who conquered much of the Western Roman Empire probably have a strong claim here. In fact, it's surprising they have never been in civilization before. They're also another household name, though not quite the brand power of a Austria or Germany, and a civilization that is not too strongly associated with any modern European nation.
The Berbers are a long standing hole on the map, and the fact that we've never had a civilization tapping the Al-Andalus part of Muslim history is also a hole that I would like to see filled. Including them as a Morocco-Al Andalus Civilization makes them the kind-of-European-but-not civilization of choice, much like the Ottomans, Byzantines and Georgians.
Asia's really the hard one for me here. Sure, Siam is an obvious choice, but it's also somewhat of a boring choice - returning from Civ V to crowd in on the South East Asian civs (Vietnam (Yes, culturally East, but geographically Southeast) and Khmer). Assyria would likewise be a returner crowding out the already existing Mesopotamian choices. I'd have loved a Silk Road cilvilization, but Mongolian Kublai happened. And what would probably be my two runaway leaders for Asia - well-known, instantly recognizable, fairly unique history, notable world impact) are both politically untenable. The safe choice kind of has to be it.
For the second Asian slot, if the Hittites can be done (language wise) I would love them, if not, I can rally to the suggestion of the Phillistines that others have made already.
In the Americas, the Guarani are both a geographic and historical hole that need to be taken care of, an important civilization, and (as we discussed in the past) arguably the only pre-Colmbian civilization whose language is still predominant in a modern American country (not even Nahuatl or Quechua come close to that). With a brand new Civilization in the South, bringing back a safe classic for the North make sense, and the Iroquois should probably be a series regular in any event.
I hesitated on Swahili for Africa because their design space overlaps with Portugal...well, a lot. But the problem is, most remaining candidates in Subsaharan Africa don't have particularly strong design space, or, if they do have one, have a strong overlap with existing civilization. With that in mind, the Swahili are the most gaping hole on the map, and they get to be the pick.
And Maria Theresa, of course, get to be our Alt Leader, for Germany since Civ 6 Germany is clearly HRE-centric.