Yucatan, Quintana Roo, and Campeche together would be a good bet to go it's own way if push came to shove, and they could possibly even have Guatemala join in to form a large, mostly ethnically Mayan-descented polity. I could see this state adopting the name of an old Mayan city-state, like Mayapan.
There would still be a fairly large country called Mexico. Even pre-conquista, the Valley of Mexico exerted a lot of cultural influence north and south and many of the various Nahuatl-descended regions would still feel a strong affinity towards Mexico, including southern states like Oaxaca. It could potentially be a regional hegemon, exerting influence through Central America if it can keep things stable.
Let's pretend that the balkanisation occurs earlier in the 19th Century. California is an obvious choice to go it's own way, though it's major problem is that it's underpopulated at the time. However, it's geographic isolation could go a long way towards creating a truly Californian identity.
The northern Mexican states and southwestern American states could coalesce into a country of it's own as well. Sparse population is still a problem, however, and it will inevitably come under the US orbit, as would an independent Texas, which is a no-brainer.
