In my opinion they could do it like Cossacks or Total War.
Cossacks had “unique” nations, but it was only Ukraine, Russia, Poland, Turkey, Algeria, all the European were designed pretty same. They Started with 14 nations and added later 8, so 22 Nations.
We had twenty-two factions in Medieval II: Total War. But the only unique was Aztecs.
3 Arabic and 2 Nomadic, everything else was European
Even there, lets take a look what we have as options for the nations.
Europe: Problem is across Europe, there is no big difference at all, Europe nations had very similar access to technology and had quite comparable combat tactics. Near Asia/Africa and Far Asia nations maybe looked different, but still had their version of same tech.
Nomadic Nations, had just different life style due to hunting and travelling with livestock. They preferred Horses and Bows, but still this technology was accessible to all other nations.
America and Far Africa, did not have access to lot fo technologies, Like Horses and Iron.
So lets say we would have by AoE4
Some African empire
Why shouldn’t other European nations not be added similar to Britain with a handful of unique units?
France is basically colour Blue instead of Red, and is famous for heavy cavalry instead of Archers.
So get a heavy cavalry Unit instead of Archer for the Elite unit.
For European Nations we have this Options:
The Byzantine Empire
The Holy Roman Empire
Also there have been several Nomadic Empires and Nations.
Why also not add them similar to Mongols with Unique units,
And so we could have easy a game with like 15 Nations at launch. That would sound way more interesting than just 6 Nations.