Okay, this might be slightly unrealistic, but still very interesting...
PoD: In 1533, the Concepción gets caught in a violent storm, and gets washed up on a beach in California. The sailors are very ill, but get taken in by the natives (Salinan people) after some suspicion. The natives' efforts to save and care for the men succeeds, and the men hold the Salinan people in high regard, due to these efforts (which had been completely contrary to their former perception of the natives as barbarians).
After a few years, the sailors have learnt the Salinan language and culture. They are fascinated and intrigued; they start to disdain the Europeans for their blind assimilation missions without knowing the native culture.
The sailors declare allegiance to the Salinan, and concerned that these people will be wiped out by the Europeans, start to give a lot of advice about farming and weaponry, and generally introducing technology to the Salinan.
This superior knowledge allows them to conquer neighboring tribes, and generally modernise while retaining culture. (Like OTL Japan)
So by the 1800s, the Salinan Empire covers most of OTL California. Its many ethnicities are united by anti-Europeanism; and it is almost as technologically advanced as the European nations. The Salinan Language is used almost exclusively among all ethnicities in the Salinan Empire now.
The Empire remains to the present day, and is treated as an equal sovereign state by Europeans.