Where would you like to see a new Supercharger in SoCal?
My number one choice would be Big Bear Lake. The climb up the mountain sucks up a lot of kWt. The many San Diego and Los Angeles tourists making their first trip up can get caught short. There are no commercial charging stations and extremely limited plug-in charging options.
Until Big Bear Lake gets a Supercharger station, I recommend topping-off at San Bernardino before climbing the mountain.