There aren’t schools to be had that come close to USC/UCLA, UT/OU, tOSU/UM, Bama/LSU/UGA. And the Big 12 doesn’t currently have any already.
No combination of UW/Oregon/Stanford/ASU/UA will make the Big 12 anything more than the #3 conference. That doesn’t mean they shouldn’t expand, but the Big 12’s fate as being outside the top-2 conferences is sealed.