Like in the name, also try being realistic. Cutoff is 1790.
Before 1790? Have the gold in California discovered 100 years earlier, somewhere around 1745. Spain immediately moves in to develop the area.
and either:
1) as the area is too far from Mexico City to be ruled from there, make the region a proper colony with its own viceroy and ties to the Spanish holdings in the pacific (the Philippines) as well as Mexico and the Pacific colonies in Columbia. These ties reflect themselves in different patterns of immigration and culture and by the time the Spanish empire crumbles California is culturally and ethnically distinct from Mexico. It will have its own revolution and become a proper independent state. As it is well established by the time the US looks to the west of the Mississippi, it is never thought of as part of the 'Manifest Destiny' to settle the country From sea to shining sea'. California having a perfect natural border consisting out of the Sierra Nevada mountains and the Arizona desert will help as well. In fact, this timeline's version of Manifest Destiny could very well be "From the snowy mountains to the shining sea"....
2) California stays a part of Mexico and is strongly bound into the cultural and ethnic fabric of the Viceroyalty. However after the Mexican revolution the region becomes increasingly difficult to govern from Mexico City, particularly with the presidency descending into a chaotic game of musical chairs with strongmen like Santa Anna. Eventually like Texas, California will revolt and form its own state. Unlike Texas, it will have enough gold and human resources to avoid Texas' descent into bankruptcy and eventual takeover by the US. The fact that California has its own natural borders as opposed to Texas being just down the coast from New Orleans does count for something as well.
Variation 2a: California could follow a policy of US/European immigration similar to Texas so the two regions face similar grievances with the Mexican strong-arm government and eventually similar independence movements in the 1830's. Eventually California, Texas and the territories in-between revolt as one and, bankrolled by Californian gold instead of Yankee family ties, the new republic of Meso-America is far less US centered. Although there have always been pro-union sentiments in both the southern republic and the US, these were never big enough for a full-out annexation but more an expression of shared culture like between the US and Canada. I don't know if this is enough to butterfly away 'Manifest Destiny', but if Canada has been able to keep its independence, then may be Texo-California could too.