Mexico's immigration into the United States is slowing down, but not stopping. Emigration (or re-emigration) from the US to Mexico is accellerating, but anyone in Texas who's had their eyes open can tell you it's always been going on. Mexico is in an excellent position to become a major world player in the 21st century. They have a technologically focused economy with a strong manufacturing base, a political commitment to education, and a geographical positioning on two oceans (one of only three nations in the world to be so situated). Although they have the capacity to be energy self sufficient, unlike Russia their export policy is not focusing mostly on exporting energy resources. Also unlike Russia, they still have a growing rather than a shrinking population.
China could get ugly in the next 50 years. All that prosperity, unevenly distributed, and all those repressed ethnic minorities stuffed under an authoritarian regime are going to bite China in the butt eventually. When it blows, expect a massive surge in Chinese immigrants.
Africa actually could be a major source of immigration, too. But the US will be competing with Europe for the best of the skilled African immigrants. With them, we share the concern of an aging population. Most European countries are shrinking in population. If it weren't for immigration, we would be too. Someone's got to change all those bedpans. We'll win the competition for immigrants, of course, because we always win. I doubt Europeans will outgrow their xenophobia before midcentury.