Technically Mexicans, Americans, and Canadians are all North Americans.
Nafta eliminated Tariffs, but Mexicans and canadians dont travel to the US without a passport. Nafta isnt all lovey dovey. There are issues with loss of manufacturing jobs in US due to cheap labor in mexico and food problems in mexico due to cheap US agribusiness food. That being said. If we were going to increase connectiveness between US and mexico and Canada more so than NAFTA already does, I do honestly believe that would require more connection between governments, maybe even a central power. So then I guess if that point ever came I could ask the very same question Im asking here about europe. Does the US, Mexico, and canada need to be more like the US is today?
Side note, Americans used to have a "manifest destiny" attitude about north america. Where we believed that everything in north america should be part of the united states. We stoped expanding after we reached the pacific, but the idea that all of North america might be united into one country has never been that far fetched an idea to americans. its one of the reasons we have gone to war with mexico actually.
And I dont see why the UN doesnt have the ability to become a world government. If we were ever attacked by aliens, they would be a good place to start rallying together all the worlds countries. Because in comparison to the threat, difference between humans wouldnt seem that big.
National identity is an acquire trait, it is something that is taught. The child of an immigrant to america will often have the same love for america and consider themselves just as american as someone whose ancestors rode over on the mayflower. The idea that a national identity cant be changed just doesnt agree with history.
The US constitution was written after we declared independece. it wasnt even our first attempt at a national government. and it was set up in a congress of member states. with representatives selected by the people in each state. We have even had states try to leave because they didnt agree with other states, and try to set up a completely different union of states. (Civil War) You make it sound as if USA started from day one as USA. In reality, the idea that the federal government was supposed to be a weak tying agent between States, (states that were mostly independent) was the main idea behind the american government for a large part of our history. it is quite easy to develop a broad national identity in 200 years.