Maybe the civs that in real life built empires after the discovery of America could have a unit to reflect that. They would be the Britons, Spanish, Portuguese, Franks and… well, as I’m typing this, I realise it’s absolutely eurocentric and people wouldn’t like it. I was going to come up with my proposed unit but now I don’t know what it could be.
HC upgraded to Arquibuser. Portuguese has an UT similar to the name though.
Heavy Pikeman replacing Halberdier.
Heavy Xbow replacing Arbalester.
Heavy Cavalier upgrade replacing Paladin for Euroasian trio - Huns, Cumans & Magyars.
Introducing “imperial” upgrades to units for various civilizations could add interesting dynamics. For instance, Franks with Imperial Paladins, Saracens with Imperial Arbalests, or Sicilians with Imperial Transport Ships could provide unique strategic opportunities. It would depend on game balance considerations and developer decisions.