Actually, I take it back. No way should New Mexico be returned, nor should Texas, Arizona, California, or Colorado. It’s ours. They want it, let them try to take it. It’s time to draw the line. NO MORE!
California, Nevada, Utah, and most of Arizona and New Mexico, plus a bit of Colorado and Wyoming, were part of the forced sale known as the Mexican Cession, in the 1848 treaty ending the Mexican War, but the Gadsden Purchase of 1853 was voluntary—the Mexicans have no claim on that territory. Giving them back the land north of the Gadsden Purchase would make for a pretty strange border. Tucson would still belong to the US, but Flagstaff would be Mexican.
I take it you haven't visited Los Angeles lately? :)