Wouldn't a free-trade and free-labor-movement treaty suffice for this purpose? I'd worry about Canada joining the US. The end state of most Anglophone nations is their universities full of Jason Ardays, their governments full of David Camerons, and their hip-fracture wait times set to 30 days. The closer culturally to the source, England, the worse the problem. Despite some of the worst foreign and domestic policies of this new century under the current US administration, we've demonstrated some amount of resistance to this corrosive nature that seems to spread through the English language.
Either Canada would immediately be the largest state and deliver a massive House majority to one obvious party or, if the provinces joined separately, an overwhelming Senatorial majority to that same party. I don't think that's desirable for the US.
Personally, Ben, I also think that US foreign policy of massive encirclement on the globe through mutual-defense agreements was a pincer move of super-strategic performance considering the ramifications it has had on our real opponents. Canada, I think, is best dealt with as a close ally whose borders represent a point our enemies cannot cross. We don't want their politics and I think we'd love to have their workers and they'd like the remittances. My position on this is like Ronald Reagan's or George H.W. Bush's.
Here's Reagan (on Mexico, but I think it works - we should expand the TN to be transferable to an LPR and weaken the labor requirement so that you get it so long as you're not a criminal):
> Why don’t we work out some recognition of our mutual problems and make it possible for them to come here legally, with a work permit, and then while they are working and earning here, they pay taxes here? And when they want to go back, they can go back, and they can cross. And open the border both ways, by understanding their problems.
To say nothing of the fact that, free of our federal government and its laws, it may well be possible to have Canada run massive shipyards, nuclear power, and drone factories in a manner that we have painted ourselves out of. Allowing for an independent jurisdiction in this way gives the West a lot of freedom.