Think of how much different US history would have been had Texas not become part of the United States. The Civil War would have been over much sooner. Many Democrat Presidents would have lost elections. JFK would have had no place to be assassinated. LBJ wouldn't have been President and there would have been no 'War on Poverty' or 'Great Society.' There would have been no $500 billion S&L crisis. All those illegal immigrants would have snuck across the Rio Grande, only to still be in Mexico.
All because Santa Anna took a nap!
The "United States" would have ended westward expansion at the Northern/Eastern Mexican border. The Louisiana Purchase would be as far west as "America" would exist.
Unless there was a war between the United States and Mexico for the land. Perhaps all of Mexico would have become a part of the "United States"; maybe it would have become 4 or 5 additional states in the Union.
Maybe some European nations (like England) would have backed Mexico in an attempt to defeat the fledgling United States.