So, I had an idea and I just want to throw it out there. Its a very broad idea and probably needs some fine-tuning but ah well.
1840s
Polk goes to war over Oregon and gets his ass handed to him. In desperation, he attempts to go to war with Mexico but Congress refuses to allow it. Only Texas leaves Mexico, the rest stays in Santa Anna's republic. The South gets louder over the issue of slavery and the unfair division between north and south.
Mormons found a nascent state in Mexican territory, and actively compete with Mexicans and Indians for their isolation and independence, so as to maintain their faith in the face of hostility.
1850s
Civil War breaks out and the South secedes. Receives some limited support from Britain, though not enough to win the war. The Civil War goes on for seven years, and at the end of it there is a strong sense of wanting vengeance on the Southerners. Dixian identity is to be destroyed, and the Freedmen put in control of the military districts. Thousands of former Confederates flee into the west, coming into conflict with Mormons.
However, the next government after the Radical Republican government after the Civil War is a Nativist government that is deeply unnerved by black rule over the South. They enact persecutory legislation, signing a treaty with Liberia to settle Freedmen on their land. Upset at being signed away to another country, many flee into the emptiness of the west, bumping into the Mormons and Confederates who have settled there before. Mexicans and Indians also come into conflict with these new settlers. The Southern Military Districts are turned into Territories.
Gold is unearthed in California, and this excites many former Confederates and the Freedmen who hope the gold will build their fortunes. The general chaos and conflict further east makes migration west very unattractive. Doesn't stop many from going, particularly those ethnic groups persecuted by the Nativist regime. Their national identities re-establish themselves on the Frontier and each nationality tends to congregate with others of their kind.
1860s
The French attempt to conquer Mexico and establish a puppet government, but it goes wrong, and the French rapidly withdraw leaving some troops behind. They withdraw to the sparsely populated north, where they act as yet another antagonistic party.
So there we go the setting for a Wilder West of intrigue and backstabbing...