Would they? Looking at New Mexico, which since 1850 had passed the whole "60,000 people = state" concept, it might take a few decades.
Rio Grande would have the benefit of being on the border between the US and Mexico. It would thus absorb large portions of Mexico's immigration to the US (instead of Texas/California/Arizona), unless Mexico averts both the Mexican Revolution and then the overall economic issues it has suffered in the past few decades--and it's still going to get immigration, since it's right on the border. And then there's still other Latin American immigration from Central America, where if we want to avoid that, we have to avoid the conditions that led to the brutal civil wars which with the US aims toward Central America, seems highly difficult. I think with minimal butterflies, the population would still be similar, if not more. They're going to get European/Asian immigrants regardless, after all.
Perhaps, perhaps not. Pointing out that Nebraska had a population of 123 thousand in 1870 (it gained statehood in 1867), Colorado 194 thousand in 1880 (statehood in 1876), and ND, SD, MT, and WA had 191, 349, 142, and 347 thousand in 1890 (all statehood in 1889). I could go on with ID and WY with 89 and 63 thousand when they joined in 1890. Or, look at New Mexico OTL: 327k in 1910, statehood in 1912. Arizona was 204 thousand in 1910, statehood in 1912 as well.
For the continental US states, the threshold which seems to trigger is that 350 thousand range. The minimum population is just a threshold; it doesn't mean that the state will immediately seek out representation when it hits that point. However, for a Rio Grande that is not admitted as a state from the start, it will likely be exceeding that threshold far earlier; it will also have a lot more interaction with the rest of the continental US, and will have a significant immigration population in time. This is compared to Puerto Rico, which, while more populous, is quite a bit more isolated from the US. So, yes, I see them becoming a state later on.
It does seem to be a bit deterministic to say as such, but Mexico is not necessarily destined to experience the same growth patterns. Again, the large population growth exhibited began within the past 50 years, not 150 years past the point of divergence. We've no idea what state that Mexico would be in absent a timeline.
Could they become that large? Perhaps; it certainly is possible. It depends on how much of OTL growth was internal compared to external. And, while it has the potential to become so populous, no one realized how populous California would become, either; the concerns of the Senatorial impact will weigh on their minds far more than the impact that the house would have.
The truth is that any bigger purchase is going to run into objections on the USA side of things, as others have noted. If not for Gadsden's unfortunate way of handling the matter, Santa Anna might well have been willing to sell more (and get more money). This would have caused domestic issues for him, but that never seemed to be his major concern. (One factor that might also help get Santa anna on board is a POD that somehow prevent William Walker from trying to annex northwestern Mexico.)
So, given that premise, a deal for more land may well be hammered out. But then... I don't see the US Senate just rolling with it. It can be done, but most likely, it requires a more fundamental POD than "the USA sends a better negotiator". Basically, a POD that can get some more support for annexation(s),
and stops Walker's mexican filibuster,
and gets another negotiator sent. (Walker also owned a newspaper for a while. Maybe the POD could be that he uses that as a platform to whip up rather broad support for annexations, instead of spending time actually filibustering? The different ATL negotiator could then just be a butterfly...)
In any case, supposing that such a deal ends up being struck, we should probably consider that most people wanted a somewhat practical border. The idea was not to annex entire states, but to draw a practical line somewhere. The maximum objective per wike would involve "most of the current Mexican states of Baja California, Baja California Sur, Coahuila, Chihuahua, Sonora, Nuevo León, and Tamaulipas"... but think that's just a bit inaccurate. I can't be sure, but keep in mind that a large part of Chihuahua's souther border ran east-west back then. I suspect the general idea may have been to extend that line from the Rio Grande to the west coast (where it would incidentally meet up almost exactly with Sonora's southernmost border.) That would then mean: all of Baja California, all of Sonora, almost all of Chihuahua, the better part of Coahuila, and small parts of Nuevo León and Tamaulipas.
More likely to get accepted by all sides, however, would be a more modest proposal, involving only Baja California, Sonora and Chihuahua. I have marked this more realistic option (and what I'd consider a realistic border for it) as "A" on the map below. The maximum reachable objective (as described above) has been marked "B".
Personally, I think that if "A" had been achieved, all of it (including the OTL purchase) would have been turned into just one state. Which would probably mean that New Mexico Territory would also become just one state. If "B" had been achieved, I see it turned into two states. (Consider that whatever extra land would be gained in the east by no means constituted the most densely populated areas of Coahuila, Nuevo León and Tamaulipas.)
View attachment 303604
Perhaps Nicholis Trist's duplicity becomes more widespread, with journalists harping on Polk allowing Trist to dictate the terms as being so favorable for Mexico, defying orders? Create the atmosphere that the US was cheated out of negotiations due to collaboration with the US negotiator and Mexico (sends claims flying about that Trist personally benefitted from what he held). That should be enough alone for the US to desire to "rectify" the makes made during the original negotiation. That provides all the motivation necessary.
Disagree on the number of states, though. In option A, there are two obvious divisions: the Baja and the inland states and regions. Furthermore, I do imagine the Chihuahua would become a state before Sonora for similar reasons as Nevada becoming a state: US desire to control the various mines in the region. There is also the question, at the time, of whether the large territory could be adequately governed from either Chihuahua or Hermosilla. After all, Sonora is much more easily accessible from the Pacific, while I think Chihuahua is more accessible overland from the East, which will result in the two regions growing apart naturally.
Mind, there is precedence. It was worried OTL that Santa Fe could not adquately govern the entire region as it was OTL. That was what lead to the OTL ideas for a north/south division, which lead to the eventual split of the territory on east/west lines in order to deny legitimacy to the Confederate state. So, as the Confederacy, inevitable this close to the Civil war even if it may not call itself that, would claim a similar region, encompassing the southern half of the old New Mexico Territory and part of Sonora and Chihuahua that are in Mexico OTL. They may even claim the entire region (Sonora/Chihuahua/Confederate Arizona) as a grand territory of Arizona, ruled from Mesilla. This would likely engender similar results as were done OTL. Except, with the Gila river as a (natural) border, New Mexico and Arizona might retain their respective northern/western bits as they don't have the southern territory.
The problem with this is twofold.
Popular sovereignty wouldn't be an issue simply because the proposed areas are pretty far south of the Missouri Compromise line. Granted, this really ended up not mattering at all once the Mexican Cession was annexed. This leads to...
The Northern states wouldn't go for this for a number of reasons. The potential for expansion of slavery was able to be avoided IOTL thanks in part to California being settled so quickly and going anti-slavery. This created the need for the Compromise of 1850. Having just averted one crisis, the Northern states aren't going to be too keen about adding enough territory that could potentially be open to slavery and the Southern states will make demands on expanding slavery into new territories. It's not a winning situation for anyone at this point. Once California became an issue, it was only a matter of time before the ACW broke out. Triggering another crisis of this magnitude so soon after avoiding the last one would just be politically stupid.
The Wilmont Proviso is what you're thinking of. It was a hotly contested one, but it forbade slavery from being expanded to any territory obtained from Mexico; this covered both the Cession and the Gadsden purchase. That was vigorously supported by the northern states, and it was already part of the law by the time of the purchase OTL. So, yes, popular sovereignty is the only way that slavery would occur so long as the Proviso remains in power (there were many attempts to alter it OTL). This could lead to an earlier New Mexico/Arizona split due to New Mexico being sacrificed as a slave territory so that the more densely populated southern regions wouldn't be. Of course, New Mexico would
detest this, so maybe not...
The offset which could distract the South is a declaration calling for the annexation of Cuba as another slave state in compensation for the expansion of free territory. Of course, a larger problem is actually
getting Cuba, but that seems to be the major prize that would distract the South.
I'll work on it a bit more
Just meant to give some advise. It might not be a bad idea to go to this link and have a look and see how it evolved OTL.
https://en.wikipedia.org/wiki/Territorial_evolution_of_the_United_States
That way, you can create your own sequence of events. It's what I did in one of my more serious map projects: Do a year/biyearly update where you change borders based on the events going on. That sometimes leads to more realistic outcomes; every border change needs some justification, even if it is just "X border is moved to Y position so that Z town/lake/mountain/port is included within the territory's boundaries." Also, look into alternate state boundary proposals. The State of Lincoln is one of those; it was a proposal to split Texas half in two, similar to what you have done, along the Colorado River.
https://en.wikipedia.org/wiki/Lincoln_(proposed_Southern_state)
Basically, write a little miniature timeline just explaining the border changes, accompanied by repeated edits showing changing borders, until you arrive at the final scenario.
