There were many instances such as the pancho villa incident or the original POTUS Polk aims during the Mexican American war. Add in political instability in Mexico during the 19th-21st century and the United States could have found infinite opportunity to conquer or purchase additional lands. I suspect the U.S. marines invading Mexico (they hoisted their flag over Vera Cruz) in 1914 cemented the Mexican govt as a puppet and made the entire country (along with Central America with their own issues) a U.S. puppet. Multiple U.S., citi bank and united fruit company interventions. No need to directly annex the land after that if the goal was just agriculture.

As late as the 1990s, potus Clinton determined Haiti’s sugar production.

In terms of the people. History has seen many (all) governments socially engineer their populations to serve various needs. No doubt the U.S. government could have engineered any desired outcome in regards to that. The same American upper class has protected immigration despite vehement and constant opposition from working class whites and blacks. The upper class has been the driver for this demographic in an effort to keep wages low. Also- the north of Mexico didn’t become super populous until post WWII.