And if this bizarro world ever happened, it would open the door for the US to join the EU by bordering an EU state and let hundreds of millions of Americans take advantage of the Schengen zone.
This would be huge for both the EU and Canada. The EU would gain a major economy and a strong presence in the Americas, and Canada would gain all the benefits of the euro and EU membership. Both entities would be stronger for it, especially in a world where the US is an increasingly unreliable ally.