but how would you translate that in gameplay ?
It's already there. When you pick a new culture at the beginning of a new era - you just choose how your society evolved during the entire previous era. Choosing new culture is just the embracing oficially all those cultural changes which were slowly going on in your empire. That's why it's cool that in your mod it will be connected to owning proper territory. No more Aztecs going Japanese even without discovering Japanese islands at all. But if Aztecs somehow discovered it, then conquered - it would be possible they would fall under heavy Japanese cultural influence - strong enough to embrace enough influence to consider themselves "Aztec-Japanese culture" - signified by old Aztec unique districts and legacy trait with new Japanese unit/district.
If I were forced to lose my Aztec homeland and play as Japan - I wouldn't be too happy about it, because in my perspective it's not about jumping from one culture to another, but evolving from one to another (as devs intended). And if people of my empire evolved - then why should I lose all those Aztec-Japanese citizens living in Mesoamerica?
@Aquila SPQR There is one basic requirement that brings deeper enjoyment to history games -- things must make at least some remote sense.
That's why I'm writing it, because I believe it makes the most sense.
In Civ even with Earth TSL map you start as American tribe in 4000 BC and spend 6000 years being American Civ. And, thankfully, we got RFC mods so that things make a litter more sense.
To be honest I don't have anything against American civ in 4000 BC, though I prefer Indigenous people when I make custom setup. I don't mind Americans, because I don't consider them "modern Americans" since the beginning. They are "people who will evolve in time into modern Americans". I'm Polish and I live in Poland right now. The closest town was founded in 1226. My village is first mentioned sometime around 1400. But we know thanks to archeology that people were living here much earlier. There was a settlement here in Roman times and there's also a cemetery from the bronze age and neolithic finds as well here (I have a flint tool myself, which I found long time ago). We can't say all those people from neolithic, bronze age and Roman times disappeared without a trace - dying out completely or migrating. Recent studies suggest that their genes are still among modern Poles. Some of them migrated, but some of them stayed here and literally became modern Poles. Now in alternative scenario in which player is able to control and guide those neolithic people into settling in one place and developing an empire - it's not difficult to imagine these are "proto-" people. "Proto-Poles", "Proto-Americans", "Proto-French". Guys who will eventually evolve into nations we know. That's how I always imagined civilization in all my Civ games. It's true that there would be no "modern Americans" without all that European influence, but still it makes enough sense to me to enjoy it.
In HK you can become American Civ in 1M/10 ways by mixing 5 previous era cultures together. Neolithic tribe became Mycaneans who influenced Romans who influenced English who influenced Haudenosaunee who influenced British who became American. Ok this makes more sense than being American Civ for 6K years, but the price you pay is that it all happens on the same land. Mycenae suddenly is American city which makes very little sense. In addition it feels silly to see Olmecs in Nubia. So this mod is trying to help things making more sense. At least there will be an option.
I'm fine with all those transition requirements (in order to become Americans you have to own American land), but I don't agree with losing territory not owned by historical new culture. That doesn't make sense at all to me. I was guiding an empire for few millenia, building a stable and powerful empire full of happy people. And now I should lose 90% of it just because I picked American culture. Why? It's not that I want to play as independent Americans limited to America only. I want to play as my old empire which, thanks to owning land in America, slowly evolved into something we would call "American culture".
Now about implementation. The basic premise of this mod is that on Earth map you have to own core American territory in order to unlock American culture. Very appealing and sensible approach, I am playing alpha 3 and enjoy my HK experience much more than with random culture changes.
As I wrote - I have absolutely nothing against that - I believe it's literally the most needed change in Humankind, the most important mod for TSL maps.
But the next step is more controversial: what to do with land from previous eras? Russians became Soviets and kept most of the land, for example. But becoming America and keeping England feels just wrong. It also feels wrong loosing all your efforts in England. I don't know how to settle this ideally. My solution is to compensate with settlers for every city lost and some lump sum of influence/money.
I'm only opposing this idea of losing all land not belonging to historical new culture. Becoming America and keeping England may feel wrong only if you look at it from "our" perspective. We know that Americans rebelled, declared and won independence and Britain did not become America. But Humankind is an alternative scenario. There is no war of independence there. Americans started to "exist" not because they rebelled against you, but because YOU and your whole empire decided to embrace values known as "American culture", because you had citizens living in America, inventing new customs and lifestyle there and you decided they are good enough to embrace them openly across your entire empire. That's how I imagine it.
Oh, and of course I don't want to force anyone to play the game as I like it, but I'd appreciate to have an option to play it as I'd like it, instead of being forced to play how I wouldn't enjoy it too much. The more options to customize your preferences - the better. In this case those who would enjoy losing old land when embracing new culture and those who would enjoy more keeping it - they all would be satisfied.