By 2050 (or the early 22nd century), I fully expect that the next great waves of immigrants would come from either sub-Saharan Africa [if the population recovers enough from the Red Plague (HIV-AIDS)] and/or the Middle East and North Africa. Those are the only parts of the world with growing populations that haven't yet made their imprint here.
most immigrants to the US tend to be from middle-income countries (Mexico, Brazil, other parts of Central and South America, Cuba, Dominican Republic, China, Korea, India, etc) rather than very poor countries because they have the money to afford to make the trip. The one exception is Haiti, and that' due mainly to proximity. Therefore I think sub-saharan Africa will have to develop somewhat before we start seeing large numbers of African immigrants.
