Well, as much as you would like to see it, it would be
a waste of space to add, why? Because Argentina is NOT
a major player in the events of world history, it exists soley
because it was a colony of spain. Now america was a colony,
but it is a special case in the massively profound impact it has
has in it's short history. Not only did america defeat Britain, the
strongest empire at the time, but it also expanded land faster then
even Ivan the Terrible of Russia did, and it did this on it's own.
Now considering as America is the ONLY former-colony to have done
such impressive things, it will also be the only former-colony to be added.
No offense to you, but this is why we will never see any civs like Canada,Argentina, or Australia as civs.