Norton I of California and Mexico
Can't do it. But I can arrange for an Emperor Norton. Having California include much US turf is right out, because it'd be a small country trying to beat a big country at war. Nor can I see how to realistically politically "liberate," say, Iowa, given the OTL reaction to such tries and the contentedness of most states. I can give it contemporary Mexico, though - a good-sized piece of turf by any standards.
Norton's wiki entry reminds me of Austin's beloved Leslie.
He arrives in 1843 instead of 1849. Instead of getting involved in business as in OTL, he decides to get in the rebellion business. Notice that his resources alone were substantial, $8Mish in contemporary $$$. He organizes a rebellion in San Francisco, corresponds with Sonoma rebels; the rebellion happens in 1845 instead of 1846 as OTL, so it comes before the Mexican-American War.
The US recognizes the Bear Flag Republic, just as it recognized the Republic of Texas, so it's a separate country.
Once the war's won, California organizes as a Consitutional Empire, with William Ide serving as the first President, and Norton I as Emperor; Norton's duties are the same the King's in his native land. Norton convinces California to stay independent because it'd be harder for commands to reach California from DC than from London to the 13 Colonies was, so it can hope to conquer Mexico, and this way California can take best advantage of its Pacific location. The Compromise of 1850 uses Arizona to pair with Texas instead. Once railroads show up, they do a base swap deal with the US to keep them from looking TOO hungrily at the Empire.
Like Texas and Mexico, there's enough bad feeling to support plenty of fighting. In Mexico, they still tell bad kids that the Rangers will get them, bwahaha. But neither the Empire nor Mexico are part of bigger associations that don't want war, and so it goes on, rematch after rematch, until there's about as much left of Mexico as Rome left of Carthage.