Someone noted folk in Canada going to other parishes because the OCA was "American" - I think I've pointed out before that such "Americanism" was not really American, but an ideological/political Americanism. OCA is still basically East Slavic in custom, liturgy, outlook, etc. Nevermind that having 'American' on something in Canada is a bit short-sighted?
As for the limitations of an American Orthodox Church - what's the worry about it not being able to send out 'missions'? The EP places missions on territory belonging to Antioch (which is Patriarch of All the East and Asia.) The EP is also on territories that Russia historically has served (and does serve: Korea, China, Japan, Indonesia, Philippines, etc.) An American Church could, of course, always help with material and personnel support (nevermind the support of prayer.) Why should anyone expect an American church to 'send out missions'? Does the Church of Poland 'send out missions'? The Finns? Estonians? Czechs? Georgians?
I think some are misunderstanding as well - if we're talking an American church, we're talking the USA. Otherwise, American missions in other lands would just be more American colonialism. Mexico doesn't need to be under an American church - it needs its own Mexican church. In fact, the Mexican church was once the National Catholic Church in that country (the part under the OCA, forced to use the OCA's recension of Byzantine rite), it is only the Greek and Antiochian jurisdictions there that are ethnic and newer. An American Church shouldn't necessarily include Canada either - Canada is a different situation, and in fact more than two cultures. Nova Scotia, Quebec, Ontario and the Great Plains all have varying cultures. A Canadian church at the least would have to have French, English, and Gaelic churches if it was truly evangelizing Canada.
There also seems to be a tendency to miss the forest for the trees. American super-culture (and the cultures that includes) are Anglo. There are other sub-cultures in America, but they are highly localized: ie, Cajun Louisiana, Hispanic Southwest or South Florida, Basque Idaho, German Pennsylvania, etc. We've already been dealing with this issue - the AWRV for instance, has both a Cuban and a Caribbean parish in Miami, FL. Evangelizing minorities however is not evangelizing a country - until you're getting the masses (which in America are White, monolingual Anglos, usually middle class, without interests in anything international, eclectic, foreign, of very respectable political affiliation, very pro-American, ... ie, not typical for most Orthodox converts - which tend to be academics, counter-culture (hippies, conspiracy theorists), or diletantes (ie, Orientalists, Hellenophiles, Russophiles, etc.) Which - nothing wrong with being those things: but - that isn't mainstream America, and you won't have evangelized America til you evangelize the mainstream. (And, a tangent: it would be a horrible failure to expect Americans to become counter-culture, academics, or diletantes to be Orthodox. That just isn't going to happen.)