Ain't gonna happen. Such a move would essentially be an annexation of Canada and Mexico. Canadians have no desire to get sucked into the United States. And, even though many Mexicans might want to live here, most are just as loyal to their country as Norte Americanos are to theirs.
But what they think doesn't matter, because it is in their governments' interests to form a larger political bloc.
I'm inclined to believe we'd be far more likely to see something like NATO + NAFTA combined instead of a new overarching State. The politicians don't want to cede their power any more than we want a bigger government.