I cant see how before 1845 is plausible
Id say a few things, in no particular order
- Deeper US defeat in the war of 1812. Less cultural enthusiasm for expansion-none of this `manifest destiny` nonesense
- Earlier discovery of the Californian gold, perhaps during the very early 1800s when the US was still quite far away and the Mexicans hadnt lost complete control of their territories.
- More intense slave debate in the US earlier on, makes the Americans focus more on solving internal problems than expansion
- Greater philosophical development of Jeffersons original statements that `the west` would be a sister republic rather than an integral part of the US proper
- Mexicans crush Texas. Texas then doesnt exist as an independent distraction causing tension between US and Mexico, which in turn makes the future of US-Mexican wars.
If all this happens, especially the gold, then you could have extensive immigration to California, but no shared borders with the US means no direct justifiable interest in absorbing California. The growth in wealth and population, and a comensurate feeling that `Californias gold` is being taken away and used for Mexicos benefit. That and the trickle through of the notion of `liberty` and `taxation without representation` could provoke a rebellion.
The result being a sort of hybridized Hispanic-Anglo American republic on the west coast-perhaps, perhaps not including the likes of Oregon, Washington, Nevada, Arizona etc