Well, if the US still wants to get it's modern borders without going to war then purchase is always an option; given the political instability of Mexico it is entirely possible that at some point one of the leaders sells the territory to build up funds in order to consolidate his position. Most of the land taken by the US was sparsely inhabited, so selling it would not be too ASB. The raw amount of land concerned makes it unlikely as a single sale, but perhaps over the course of decades several different leaders sell a couple chunks of their uninhabited northern lands to the US, until the border ends up more or less where it is now.
There could also just be a continuation of the sort of "settler imperialism" that produced the Texan revolt in the first place, with large numbers of Americans settling in Northern Mexico and ultimately rebelling, gaining independence, and then promptly requesting US annexation. That could also contribute to making Mexican leaders more inclined to sell land to the US; as it would ultimately be lost anyway they might as well make some money off of it now.