As a fan of Portugal both in AoE2 and 3,and crossing my fingers that they will make their way into the game eventually, here is what I would like to see in AoE4 considering their history, and their AoE iterations:
Unique Units: Caravel/Nau (Carrack)
This one is a given.
Instead of organ guns, Breech Loading Swivel Gun, still anti infantry.
And above average arquebus, thus maintaining their Naval/Gunpowder focus.
Depending on how the ages are divided, having good Crossbowmen and Good Light Cav would also go along with their Reconquista origins.
Now I’m a fan of the feitoria, but I would like to see it as a fortified forward base, that you could possible garrison with a few units, and would make more resources the further away it is from the city center.
This could open up for risky and profitable gameplay options, or it could backfire terribly.
On the Hero/Commander unit side, one thing that criminally overlooked in Iberian history is the use of the double handed Greatsword Montante, and I would like to see the Port leader rocking one of those (like in my forum avatar), while giving a buff to infantry units.
This is just a little brainstorming I’m doing.