I can't think of a realistic way to get the Rio Grande to become an actual, functioning nation. The movement was rather tiny, the actual states supplied troops to the federal army to crush them, Texas didn't even publicly support them. To be quite honest (as this is the second Mexican secession thread I've seen recently) I think Texas was the only Mexican state with any chance whatsoever of successfully seceding. They were the Mexican frontier, populated largely by foreign immigrants who were completely different from the other Mexicans in every way, and pro-slavery and non-Catholic in an officially Catholic country that was not kind to slavery, this meaning that their views were totally different from the establishment. And when I say frontier state, I mean even other Mexicans didn't know much about the place. One reason for Yucatan's secession was that the people did not like being drafted to go put down a rebellion in some "far-away and very cold" place up north.