The Plan de Iguala goes into effect in Spain. This results in a compromise between the monarchists and republicans in Mexico, with the nation having an emperor but not one to directly support like with the actual Mexican Empire. This leads to more stability and Mexico holds onto Central America and Texas; it also manages to fend off any further land grabs by the United States. This leads to a strong and prosperous Mexico that grows into more and more of a democracy, while the monarchy becomes a mere symbol like the British monarchy in Canada, or ends depending on developments in Spain. This gives no reason for Mexicans to immigrate enmasse to the United States, given the Mexican Empire would be one of the richest countries on Earth.
Vive Mexico!
Vive Mexico!