This sounds very similar to Age of Mythology. In that game, there are only three civilizations: Greeks, Egyptians and Norse. Where’s does it branch? To start the game, you have to choose one major God - there’s three for each civilization. Comparing it to the current topic, we would have: European, Asian, American, and the “subs” could be: German, English and French; Chinese, Japanese and Mongol; and Aztec, Mayan and Inca, respectively.
In Age of Mythology, the three civilizations play very differently. All of their military units are unique. Norse don’t even have archers. The major God chosen at the start gives different bonuses, so the same civlization will also have some tweaks, and to advance Ages you also have to chose minor Gods. Age of Empires III mimicked this with the Politicians.
If we have the same system in Age of Empires IV, then it would be, for example:
Major civilization: Europeans. This will give you the unique unit Longswordman, and access to bigger or stronger Castles.
Subcivilizaton: British, which gives you the Longbowman and faster working Archeries; or French, which gives you the Chevalier and 20% more HP for Cavalry.
Major civilization: Americans. This will give you a lack of cavalry (and this needs to be balanced properly, not like they did with the Eagle Warrior, which is… not very good). I think a good mechanich for the American factions would be to get 3 Villagers for the price of 1.
Subcivilizaton: Aztecs, which gives you the Jaguar Warrior and faster creation time for all military units; or Mayans, which gives you a unique Monk.
Now, there would be a problem with the Asian factions, because I’m putting forward the idea that Mongols shouldn’t build walls , but the Chinese have built a really long wall in real life. So, they would be too different to belong in the same major civilization.