This is very true; "Therefore go and make disciples of all nations..." He did not say "make disciples of all provinces, dioceses or regions, did He?
Why can't Canada, the USA, Mexico, Brazil, etc.. be considered nations in the Biblical sense?
I would say it is because they coincide with political borders just as much as the Roman set-up. It's not like anyone's advocating for a "Cherokee Orthodox Church" and a "Navajo Orthodox Church" and an "Appalachian Orthodox Church", etc. As I understand the biblical term for "nation" (and perhaps I'm wrong), the US has any number of nations within its "national" borders. The other countries you named have the same issue. Many large countries can say the same. Unless we are comfortable with a million, small, autocephalous Churches, I think the argument in favour of "national Churches" needs to be made apart from a reference to the Great Commission.
It would be more reasonable, IMO, to argue that the organisation of the world in the 21st century doesn't follow the same pattern as the Roman Empire, and so we should apply the latter example of accommodation to the world political structure to the world as we know it today. If anything, it was that principle, and not the exact order of cities, that was enshrined in canonical legislation. Apply it today and watch the sparks fly.
As Isa, our resident historian, pointed out, there were at least six autocephalous churches in the Roman Empire. Nonetheless, the churches still ended up organized to suit the political realities. That said, I think that the Lord did not equate "nation" with a political entity--I think He was referring to a practical reality: a nation is normally composed of folks of certain ethnicity, speak a same language, have cultural similarities, share aspirations and occupy a defined place in our world. Empires usually are formed by expansion and thus contain various nations in their boundaries. The Roman, Ottoman and Russian Empires were certainly so, and very much like the hyphenated Americans, one had two identities: one that defined political allegiance and the second that that defined the real nation that one belonged to. Thus, in the Ottoman Empire, even the Turks would be hyphenated--Ottoman-Turks, to distinguish them from Ottoman-Greeks, Armenians, Bulgarians, etc.
In the modern age, it is usually not the case that most politically defined nations contain many radically different ethnically defined nations, with the UK, Canada, Russian Federation, and Switzerland being the exceptions to the rule. Since we are close, let's take Canada as an example of how a politically defined national local church can successfully serve the English and French speaking sub-areas of the country. I would think that if there is a future Orthodox Church of Switzerland, it would have a tri-lingual aspect to it. In the United States, that is also possible with parishes that are bi-lingual in order to serve those recent immigrants who are not yet proficient in English. Certainly, in the transitional phase, the current ethnic dioceses would be subsumed into the local church without any discernable changes in the lives of their parishioners. The OCA and the GOA are proof that such arrangements can work. However, there would be (must be) a different strategic approach for any autocephalous church in North America: the mission would be to grow the Body of Christ, period. That would not obviate the requirement to respect the cherished ethnic traditions of existing cradle Orthodox. It means however, taking a bold step forward in acknowledging that the local church will have a great number of parishes whose liturgical and parish practices echo various Old World churches and nations, along side of purely American parishes that may or may not keep such Old World praxis and thinking. It may be that even ethnic churches will have outreach events such as a Mediterranean Festival, rather than separate Greek or Lebanese Festivals. Somebody mentioned an Appalachian church, something that would be a good thing indeed, if they were to have a Appalachian Food Fair for community outreach.
The point that I am slowly making is that numbers sometimes talk and here are the numbers for the United States:
Population: roughly 317 million (2014 estimate)
Pew Forum on Religion & Public Life / U.S. Religious Landscape Survey:
Evangelical Protestant Churches 26.3%
Historically Black Churches 6.9%
Other World Religions <0.3%
Mainline Protestant Churches 18.1%
Jehovah's Witness 0.7%
Other Christian 0.3%
Other Faiths 1.2%
Don't Know / Refused 0.8%http://religions.pewforum.org/affiliations
US Census data, race/ethnicity percentages (2010)
As the table at the link below shows, except for Aleuts/Eskimos, no other ethnic group normally associated with Easter Orthodoxy is mentioned.http://en.wikipedia.org/wiki/File:Census-2000-Data-Top-US-Ancestries-by-County.svg
Thus, a truly autocephalous church of the USA has the potential of making ethnic differences meaningless, even if all reasonable efforts are made to respect and preserve such differences. This would hold true no matter how broadly or narrowly one defines the new local church boundaries. I submit that some folks believe that the only way not to get lost in an "American" church would be for the current ethnically-based jurisdictions to oppose autocephaly and evangelization with all of their might.