I would not be surprised to see the United States eventually grow to include all of North and South America. Certainly I don't see that happening in our lifetimes, but over the next 200 years, who knows? Even Cuba may become a U.S. state at some time in the future. Wouldn't that make Castro roll over in his grave!
So long as the new states adopt our Constitution and Bill of Rights, I have no problem with the United States continuing to expand. Many Americans were horrified over the Louisiana Purchase and some states threatened to secede over it, just as you threaten to emigrate (they feared that they would lose their influence). But it all worked out rather well in the end.
Not quite so fast. That's the plan, amigo -- their plan. Do a search in a reference site on "plan" and "San Diego" and "Mexican-American" or "Chicano" or "pachuco".