I can't think of any benefit for the two countries to become one that can't also be achieved through treaties and trade negotiations. Using those methods would allow each country to keep their ideological differences because those differences work well in each country. The Western portion of Canada probably has more in common ideologically with the Western and Southern portion of the United States, whereas the Eastern portion of Canada is much more aligned with northeastern United States values.
The one thing a Canada does have is a better name for a country. Because it's just Canada that's it it's just a simple name. The United States of America has one of the lamest country names there is. It's not really a name of a country it's a description that's been adopted into a name, because we're so disagreeable we couldn't even agree on a name. Columbia would have been a good name but we missed out on that one didn't we.
Such an entity could compete with any country on earth, from competing for cheap manufacturing wages and costs, (as Mexico is doing quite successfully right now) to competing on quality.
English, Spanish, and French are three of the most widely-spoken languages, So most people on earth could find someplace which felt at home for them linguistically, or even if they didn't emigrate here, they would find vendors and customer service people who spoke their language.
It would be phenomenally defensible: protected by two oceans and the arctic, it would have no hostile nations on any border, like India, China, Russia, and the EU do. And therefore it would be a region where people feel safe to invest their money.
And its currency would be the world's reserve currency. It would have by far the best natural advantages of any economic union in history. If only we could persuade our fellow Americans that Mexicans aren't our enemies, and that we won't all just drop dead if we adopt single-payer health care system.
Euratom for easying the trade of niclear material and monitoring the radiation in the membercountries.
Ecsc similar to euratom for coal and steel. They also brought an democratic legitimated supranational court that is now the ecj
And last but not least ec and eec, the brought free trade and some of the standards, like minimum size of apples.
They trade with US. Being a Neighbour of US, they get protection.
Canada gets to have the good parts of US without being US.
It would be a stupid move for Canadians.