They’d probably join Arizona. Full casinos if they’d join Nevada might be a problem
There would be a slight problem of setting a boundary in the San Bernardino-Fontana conurbation. Boundaries would be modified: with the exception of Twenty Nine Palms give all the military posts to California. Joining Arizona would be the easier (but still very difficult) path than that of creating a new state.
This would be their best bet if Arizona would be okay with it. Parts of San Diego County or the whole thing might look at doing it too. In the old days when SD Cty was red I'd say yes but these days it's too blue. I'd want to see the percentage of red vs blue voters in SB Cty too.
NorCal and inland central Cal has been looking at joining Idaho.
By joining another state they don't change the stars on the flag or the balance in the senate.
Really the only bad section of California is the coast and inland about 25 miles. Take that strip away and it gets red really fast. It's easy to see why so many Californian's are fed up with the unfriendly laws, policies, and super high taxes and cost of living.