This begs an interesting question... How would our "northern neighbor" (the USA) react when thet see a stronger, more united Latin America?
Depends on how strong Mexico is in this TL and whether or not Mexico can expect help from Gran Colombia and/or other Latin American countries. For much of its history, the US did not have the ability to project power in South America, and was more concerned with obtaining a Pacific coastline (eventually obtained through war with Mexico) and developing the resources of the land that would become the Midwest and Great Plains. So South American states have plenty of time to get themselves together to the extent that they would be more than capable of competing with the US for influence in the hemisphere. The Americas would not be the backyard of el Coloso del Norte.
But Mexico is a different story. Personally, I think the success of the US is inversely proportional to the success of Mexico. Mexico will have a hard time keeping all its northern territories with a United States united behind the idea of Manifest Destiny. There will inevitably be a confrontation between the two of them, and the fallout will be disastrous for the government of the losing country. In OTL, Mexico was humiliated and shown to be ruled by weak, incompetent men largely motivated by self-interest, followed by decades of civil war, foreign intervention, and dictatorships. In an ATL where Mexico wins, the US no longer has the kind of territorial expansion it needs to avoid the discussion on slavery, and an earlier Southern rebellion means the *Union and *Confederacy are much more evenly matched.
So I can see it go one of two ways. In a scenario where Mexico progresses much like OTL but South America remains more unified, the US still expands and competes for influence with Gran Colombia or whoever in Mexico and Central America. In a scenario where Mexico gets itself together, then it has a cordial relationship with *Canada and is the major power in North America, while the US descends into regionalism and civil war. We could see a scenario where Latin American countries exploit the former US (though for what resources?) much like the US exploited countries in Latin America, with *Canada being a lot like Argentina was OTL: a country far enough away and in enough competition with Latin America to be able to lead the charge diplomatically to protects its fellow Anglos.