its hard to really fully determine what the course of history would be if there hadnt been that Mexican-American War, but i feel that the same territory (or roughly the same) would have ended up in United States hands anyways. There were already many many Americans emigrating the California, Utah, New Mexico, Arizona etc, and as has been proven in times past, Americans usually want to be American, or at least not under someone else (meaning either under the United States Government or Independent, as Texas tried to do).
I think the most that would have happened is a delay of 1 or 2 decades. Of course thats also looking at it from the US point of view, it could have made things very different from the Mexican point of view. If i remember correctly, Santa Anna was deposed for a time after the war due to a poor popular sentiment towards him. If he hadnt been soundly defeated in the Mexican-American War perhaps he would have tried to build upon his reputation of the Napoleon of the Americas, either by conquering neighbors to the south or trying to fight in what is now Canada or Texas/US (which would be basically the same war as did happen).