A stronger Pro-Confederate support in southern California. There was small support for South California to secede and join the Confederacy but It didn't get far due to low support
This, and to seal the deal, have H.H. Sibley do a far better job against the Union in the West. Hell, maybe recruit someone else to lead the campaign, as well as send more soldiers for it.
Can the CSA hold it, that's highly doubtful without foreign intervention (the usual British and France intervention).
There's also what I've seen with the Confederate States purchasing/conquering Sonora/Baja California to get their Pacific Coast, which kinda makes sense. But I'm more interested in what's basically a New Mexico Campaign wank, since Sibley aimed to conquer California, or at least Southern California. There were indeed visions of slavery being established there. It isn't the most suitable terrain for it, but that was always a goal.
And I suspect the Confederate States's response to Chinese immigration would probably make the OTL US look good. But as long as both States don't have a Chinese Exclusion Act, then immigrants will still come, if only to go to whatever country is doing better, which I suspect is probably gonna be the USA. Both states will need a Chinese Exclusion Act to really change things up. In which case you'll see the OTL examples of Chinese coming in from Canada and Mexico, just changing things up to account for California being divided (no way in hell is the CSA gonna get ALL of California in whatever treaty/ceasefire ends the ACW).