What if expansionism in America hadn't occurred?

  Slightly confuses me about Mexico.  Are you asking about who might end up with Texas - from the comment about the oil?

  Less access to the Gulf of Mexico than access to the central region of N America.  A hell of a lot of resources there and it also links up with Canada very nicely economically.

  I think the primary problem was crap leadership for much of the period, especially but not only Santa Anna. Also it seems to have had attempts at establishing a strong centre which really upset the outer provinces. 

  Possibly in TTL where pressure is likely to be less there might be the chance for it to get its act together.  However sooner or later whoever has Louisiana, probably but not necessarily Britain, that's going to start filling up and some settlers are going to look further west.  Texas might not look as attractive as OTL although you might have problems with disgruntled slave-owners if in the same period Britain is moving to ban slavery.  [Mind you I think a factor in the OTL Texan revolution was that a lot of the American colonists have brought slaves in despite this being against Mexican law and the agreement they had made for land.]  However once gold is discovered in California then unless Mexico has a very good hold on it, it likely to be swamped by Anglos of one form or another and probably lost as a result.

  Which raises an interesting question.  What happens to slavery in this US?  If limited to east of the Mississippi its going to fill up earlier, especially since cotton plantations are so land hungry.  Hence not only is there likely to be a clash over it but it would probably happen somewhat earlier.  Which good be good for the south as an earlier war is likely to reduce northern industrial superiority and also if before railways get started overrunning the south would be a lot hard.  On the other hand, without expansion to the west there's going to be a lot more small farmers who will be competing with the plantations for land.

But didn't California sought independence only due to American filibusters? California's population was very thin previous to the Gold Rush. I believe California would remain a part of Mexico without US interference. 

But... California didn't had the population nor the motive. The US incited the Californian independence OTL, by sending American settlers to increase the population in the US's favor. They separated from Mexico only to immediately join the US. Texas may be independent, if American settlers come to the region on their own. Texas may be successful, but I really don't see any chances of an independent California without the US.

  There is likely to be an outflow of people from such a deeply introvert a US as some look to find more opportunities.  How big an outflow would depend on the circumstances, which would probably change at different times.  If they move into Mexican areas and Mexico is as badly governed as OTL then you could see some breakaways which could well be successful.  However if Britain add Louisiana to British North American many of any such surplus could end up there.  [Whether this might lead to conflict there and what results would be another important issue].

  If Louisiana becomes British then there is probably less chance of American succession attempts on Mexican territory as they will be less capable of reaching such areas and establishing links to the US.  Which could mean in any successful successions by American colonists would end up as independent states.  As I said previously I doubt Britain would make a bid for Texas, unless this ended up as a result  of disorder in Mexico, as Louisiana is likely to take a while to fill up.  However once gold is discovered in California I can see Britain/BNA competing in terms of control when the influx of gold-miners means it probably ends up breaking away.  Especially if BNA throws up an equivalent to Cecil Rhodes to encourage expanding British control.  The region is going to be too wealthy and under those circumstances the largest group, if not the actual majority of the Anglos brought in by the gold rush are likely to be from the British empire.

  If Louisiana was to stay French rather than British after the end of the Napoleon wars, or equivalents then the sheer bulk of the population in the US - relative to Louisiana anyway - could end up with a lot of Americans moving into the area and resulting in pressure to change control.  Whether, to meet the need of the scenario an isolationist US the latter rejects any union with a breakaway Louisiana or the 'American' settlers decide they want a totally separate state I don't know.

  Well technically the Americans OTL did move into much of 'Mexico', its just that they ended up taking the territory from Mexico.  After settlement in the case of Texas and pre-settlement with California and other areas.  There were other areas of northern Mexico that weren't taken over by the US but I think a lot of them are pretty barren, such as Baja California. 

  I think its likely Louisiana would end up British in this case but if it stayed with France and also pretty thinly populated then I could see a lot of Americans moving there for the land.  Of course its likely to be a lot more difficult taking Louisiana from France than taking Texas from a relatively weak and poorly led Mexico.  However if the majority of the European population was English speaking from the US or elsewhere there would be likely to be tension between them and the French speakers, especially if there were attempts to impose the French language or other aspects of French culture on them.

  I think California will boom sooner or later because a) gold is almost certain to be discovered at some point and b) at least for northern Europeans it has a better climate than areas to the south or east which tend to be hotter and drier.  [If only the ground would stay still!! ]
 

  If Louisiana became 'American' after prolonged settlement, or was predominantly French and broke away from European France it would probably be a republic.  It might just change its name if it was an American dominated one, or possibly even if it was a French speaking break-away from a monarchical France.