I don't understand why everybody is so against Mexico joining our theoretical NAU. Sure, the first decade might hurt a little, but in in the long run it would be beneficial to both Americans and Mexicans. Especially Mexicans. And why stop there? Central America would be an obvious next step. I'm not sure if going into South America would work, but maybe a few decades after Panama joins up...
Language is the obvious problem, which is why Quebec should stay their own country as well. The fact that Mexico is a much poorer nation has to be taken into consideration as well, but that can be reconciled within a decade or two as you mention.