So one option is to have a linear Civ tree and simply change the name of the Culture group at certain stages. Or let the player choose in certain Eras which Civ to become. So you'd start with a generic group (like idk proto-europeans) and then the player can choose which (european) Civ to become.
Count this as the simplest, but also the least interesting, way to do it. For one thing, not all Into-Europeans were alike, and by no means did they all turn into European culture groups: Indo-European is named such because it includes groups that moved into areas from Europe to India, and 'spawned' such different cultures/Civs as Germans, Romans, Greeks, Persians, and most of the Civs/groups in India.
Maybe you could have a fun name mechanic where for e.g. Spanish + Norwegians = Spanish Norwegians or Norwegian Spanish. A silly and terrible example but I think you get the idea. And depending on the mixing and merging (which is influenced by trade/social policies etc) your Civ will change, in regards to language, ethnicity or even unique units etc.
Not so silly, when you do the research and consider the variations available and that actually happened. In fact, although about 1500 years earlier, the Celts who migrated into northern/central Spain mixed with the native 'Iberians' and spawned the 'Celtiberians' who fought the Romans for several hundred years and from whom the Romans 'borrowed' several of their own 'iconic' weapons: the long "Spanish" sword, the short Gladius, and the heavy throwing javelin, the Pilum.
'Normans' were 'Northmen' settled in northern France - another 'mixed' group for sure.
And some Cultures/Civs we think of as Specific are actually mixtures of even earlier Cultures: the Greeks migrated into Greece as Indo-Europeans, and Greek 'Culture' is a combination of Indo-European elements (Sky/Thunder Gods like Apollo and Zeus, battle carts/chariots drawn by horses, a Warrior 'class' that 'elects' the leader, etc.) and elements from the earlier groups living in Greece since the Ice Age (Agricultural Earth Mother Goddess like Hera, megalithic masonry, - even place names like Athens, which is a non-Greek word, linger). There are almost no modern 'Cultures' or Civs that do not have Influences on them from some other Groups, Cultures, Religions, or military and technological 'borrowings'.
Another way to have a more dynamic Civ would be to more or less disconnect unique units/buildings from starting/pre-defined Civilizations and instead they can become accessible to the player depending more on culture mixing, settling, trading, warfare etc.
If for e.g. you start or settle up in the cold north (maybe walking there with your Hunter/Gatherer units) and when you found coastal cities and imorove tiles close to the coast, then your culture could become either Britons, Norwegians etc. After meeting some basic conditions you could gain access to those unique units (maybe through applications/projects). You could develop Longships if your Civ has demand/need of good seafaring vessels and you have neighbors with rich cities, trade network etc. Attacking your neighbors coastal cities/trade routes could award you naval tech pts and allow applications like Longships and Berserkers as well as influence the culture or development of your Civ.
A lot, per your example, depends on Resources Available. Thinking on something like this back in early Civ V days, I hypothesized a 'build your own civ' system in which your Starting Position would have three sets of Basic Resources that governed how you began:
Water - sources:
River
Lake
Oasis
Coast
Food - sources:
Fish, Shellfish, Whales from rivers, lakes, or coastal seas
Plants with Gathering or Agriculture: Wheat/Oats/Rye/Barley ("Grains"), Rice, Potato, Maize, Berries (later, "Wine")
Animals with Hunting or Husbandry: Deer, Cattle, Sheep/Goats, Horses, Bison
Building Materials - sources:
Clay
Timber
Stone
(Marsh) Reeds
Even with the very basic list of Food Sources here, that's potentially over 200 variations of Starting Position Resources, which is more variation in 'Civs' than the game can handle. Add the fact that usually there will be a choice of more than one Basic resource of one or more of the three types, and the potential variations would be enough to handle all possible 'starting cultures'.
And, a lot of the Unique attributes of various Civs were not the result of resources/situation, but of resources available combined with Resources Absent. The Scandinavians were essentially the same Food From Husbandry and Agriculture group as the rest of northern-central Europe BUT, especially in the north and along the mountainous coast of Norway, agriculture was marginal at best and the animals suitable for Husbandry limited, so the Sea became the primary food source, and it was not a relatively benign set of rivers and lakes but the open, notoriously stormy North Sea - teaming with fish to eat and conditions to kill you quick in a reed boat. To exploit the sea resources, the Norse had to build very sturdy timber boats - and had plenty of timber handy and no real alternative Food Source once the population got past the numbers that limited agricultural land could support.
Result: "Plowing the Auk's Meadow' in the iconic Norse phrase, in sturdy wooden longships and knorr 'roundships',with Celestial Navigation to bring those fishermen home. Couple that with access to Iron deposits in abundance and lucrative Targets along the North Sea coast in Britain, Ireland, Germany, France, etc. and the well-armed and armored 'Norse Raider' followed ...
But note, anywhere in the world you have mountains coming right down to the sea, in places as different in other ways as northeastern Spain (Loret-de-Mar), the Dalmatian coast of the Adriatic, or Norway, the result is a 'Pirate Coast' where the local inhabitants make most of their living from the sea by fishing or raiding, and cannot be constrained by any central government on the other side of the coastal mountains...
Finally, a lot of the Unique Attributes of Buildings, Improvements, and Units are a combination of both Resource/Social/Cultural/Technology combinations AND special needs of the situation for that Civ and Pure Randomness. Classic Example the Roman Legion, which started as a basic Decimal Organization spear-armed phalanx of personally-armed Citizens absolutely indistinguishable from any classical Greek City-State. Turning that into a primarily sword-armed flexibly-organized organization of heavily-armored and extremely well-trained men took an almost-unforeseeable combination of borrowed weaponry, changing Needs and Enemies, (if the first Roman opponents had been Scythians instead of Samnites or Etruscans, you can be pretty sure swordsmen would have never been their military priority, and the 'Legion' would have probably developed into a Byzantine-like organization of light and heavy cavalry with a few infantry auxiliaries) and Social Policy/Cultural peculiarities, a combination not found anywhere else in the world historically.
I quit playing with the idea because I couldn't see how to recreate the absolutely specific requirements for each Unique and at the same time keep players from 'recreating' those requirements because they were playing as Rome just to get a chance to field Legions, or build early Paved Roads. I'm not so sure it's impossible on further thought, but part of the 'solution', I think, would be to create Classes of Unique (Units/Buildings/Improvements). You might 'select' a Class of Unique Unit to 'achieve', for instance Light Cavalry because you have Scythian/Hun/Hsung-Nu/Commanche neighbors, but within that 'Class' there could be:
'Tarentine' style (Greek/Roman light cavalry with shields and javelins)
'Thracian' style (light cavalry with sword, shield and javelins)
'Bactrian' style (Near Eastern light cavalry with bows and javelins)
'Persian' style (Persian light cavalry with javelins and thrusting spear)
Prodromoi (Macedonian light cavalry with long lance)
- and these are just examples from Classical Era Europe and the Near East, but indicate the variety that could still be introduced with this kind of system. In addition, of course, a common Counter to enemy/barbarian raiding historically was to simply hire as mercenaries some of the same type: the Athenians hired Scythian horse-archers, Alexander the Great incorporated several thousand Scythians into his army, Rome and Byzantium both hired Hunnic Cavalry, the Chinese hired the Hsung-Nu and their successors, and as late as the 19th century CE the US Army hired Apache 'light infantry' to hunt down Apache raiders...
