It would either draw Texas into yet another war with Mexico with uncertain results, or it would expand peacefully into those lands controlled by Rio Grande. The most likely result would have been war.
If war, we have several possibilities.
1) Texas wins big and forces Mexico to recognize its possession of the territory of the Republic of Rio Grande.
2) Texas wins a less than decisive victory and gains control of some of the lands, but not all.
3) Texas fails to defend Rio Grande, but Mexico agrees to recognize Texan possession of lands between the Nueces and Rio Grande Rivers.
4) Texas loses. Mexico not only reconquers Rio Grande, but forces Texas to accept its border on the Nueces River.
5) Texas loses big. Mexico defeats Texas and forces further concessions on Texas. It is unlikely Texas will be integrated back into Mexico, but we can expect Texas to cede lots of land, particularly out west which is in practical control of the Commanche.
The most likely result is #3. Texas has certain advantages over Mexico, but also disadvantages. I don't expect a repeat of the Battle of San Jacinto. Texas still enters the United States.
If so, this removes the major casus balli of the Mexican War which happened as a result of the disputed area between Nueces and Rio Grande.