The one school in the West that makes a whole lot of sense media market wise as an addition to the PAC 12 is San Diego St. However, California’s design was to have the UC Schools be the research institutions and the State schools were non-research institutions. Over the years, that has blurred as many of the State schools have crossed over into the research side. Still there is that stigma and the UCs will fight it.
The other issue is what other school they’d take with SDSU? Boise St would never make it based on academics and non research status. Same with BYU.