I voted for the Spanish because to me they have the most distinctive history that covers the historical time periods at the heart of this game.
Their national bonuses should be some kind of gold looting bonus (or caravans that produce more gold). I would make their special unit an infantry unit which received double bonus of any Armor Blessing or Weapon Blessing (including both), to capture their missionary zeal at the height of their powers.
The Egyptians would be an obvious choice. I might among other things give them double vault capacity (to represent the treasures hidden in the tombs of pharaohs). Their farms might receive production bonuses due to the flooding of the Nile.
I am very much against having "Americans" unless they are Native Americans. However, it would be an interesting twist if there were nations (like the United States) which were only available as Revolutions after obtaining a correct historical age. (I.e. the US would become available in Gunpowder Age; Brazil might be another such nation). If they were to implement such a strategy I would hope there would be a discount for revolting to a modern country, or else that every player would get one free Revolution, which might be used for such a change. Other nations that might be such "revolution countries": Australia, South Africa, Ottoman Empire / Turkey, Byzantine Empire.
Or, what if each regular nation had one "revolutionary" offshoot that you could switch to at a certain point in the game -- so that British players could revolt to become the U.S.; Spanish players could revolt to become Mexico; Greek players could revolt to become Turkey; Roman players could become the Byzantines or Italy.
Just a thought...