> No, that's actually off limits, by treaty.

They have to amend the treaty anyway to allow a new member state to join, so why couldn't they just amend it to remove that limit at the same time?

What really matters here isn't the letter of treaties, it is the political reality that it ain't happening. Canada's economy is very tightly bound to that of the US, joining the EU would be erecting all these new barriers to Canada-US trade which would hurt the Canadian economy, and the reduced barriers to Canada-EU trade wouldn't be enough to counteract that. There is no escaping the geographical reality that the US is right next door, the EU an ocean away.