I see the name "New Frontier" as a new way to give out DLC and make this a test run for future games.
I don' know we'll see. I'm assuming once we reach 50 civs they won't include any more and that was the initial goal I had in my mind for Civ 6 anyway.
It's certainly possible that this was only intended to prime players for whatever the next game is. Although I doubt that game would be Civ 7. Players wouldn't want to reset to ground zero and buy the same few dozen civs all over again without a massive mechanical overhaul, and at least with VI we had Beyond Earth giving us something different to play with for a few years and make us forget that VI was just V all over again. But maybe the next "Beyond Earth" experiment could try to use that release model.
I'm torn. On the one hand, if they are already releasing civs with vanilla mechanics and "matching civs to playstyles," it feels like they are out of ideas and eeking out the last suresells. On the other hand, I've observed that VI is a great foundation for a legacy style game, and we are missing an economic victory, and New Frontier could easily be seen as filler content while they polish out more complicated mechanical additions for future content.
I also just don't feel like the game will be "complete" after New Frontier. The biggest things that stand out to me:
1. If we only get one North American (Maya) and one South American (GC) civ like in previous expacks (Cree/Mapuche, Canada/Inca), then we don't get a western American civ. The Navajo (or maybe the Apache) seem like such a logical fit for the game, and if we have Cahokia it seems fitting for us to get a Pueblo and/or Haida CS. But that can't happen this season if the other six slots are dedicated to other parts of the map.
2. We have Khmer, but no Vietnam/Hanoi, Burma/Pagan, or Siam/Bangkok. I actually think Burma ought to be a civ in its own right alongside Vietnam, and yet we will almost certainly not get both this season. But even if we only got Pagan or Hanoi, that still leaves three very large, influential empires that will likely not all get civ/CS representation in New Frontier. Until we have all three, I don't think SE Asia feels very complete at all when we have Babylon
and Akkadia, Ireland
and Wales, Swahili
and Madagascar, Kanem-Bornu
and Nok, so many other empire-dense regions fully fleshed out by city-states while we have nothing but Khmer on the SE Asian Mainland.
(2.5 We also don't have Helsinki or Copenhagen or Vienna as CS's yet, and it would still be nice to see a Lhasa CS, maybe even Yakutsk to give us some Siberia/Sakha representation, or Honolulu or T'ui Tonga to help fill out the Polynesian triangle. Point being not even the city-state list as a whole feels complete yet.)
3. To a much lesser extent (because we already have Preslav and Antioch) Bulgaria deserves a shot, and yet the science slot has already been taken by the Maya and players really want Byzantium. I don't see both Byzantium and Bulgaria happening this season, but it's also one of those regions where--if I take VI's "gap-filling" philosophy literally, which although could be wrong still very much seems to hold true so far--I don't see it being completed this season, but I also don't consider the game feeling "complete" without both civs.
4. To an even lesser extent (because we have Fez and Muscat and Zanzibar added in expansions, not even in the base game), I think Morocco/Berbers/Numidia and Oman/Swahili do a very good job of filling large geographic, cultural gaps on the map. Again, I think the game wouldn't feel very complete without them, but I also don't see both happening this season, although we do stand a solid chance of seeing one or the other.
Tldr; I could easily see a "complete" version of the game having: Navajo/Apache, Vietnam, Burma, Byzantium, Bulgaria, Morocco/Berbers/Numidia, Oman/Swahili, in addition to likely candidates like Portugal, Timurids/Mughals, Assyria/Armenia/Palmyra, Italy. Given how holistic design appeared to be prior to New Frontier, I haven't yet abandoned the notion that the developers have a "full picture" in mind. And, if that be the case, there are just too many gaps on the map with potential for the picture to be "completed" with only five more civs after Ethiopia. I would not be surprised in the slightest if the devs try to stretch out another set of DLC after this season to cram the last few ideas in.