BYU had discussions with the PAC before, with the Sunday conference games being the sticking point. Obviously they are nothing close to ND, but they do have something of a worldwide brand. They are not immune to money either, and when their church leadership inevitably becomes slightly more progressive the issue will be revisited again.
The now defunct Boise St wrestling team used to compete in the PAC, and yes they’re much more a reach than BYU, but there is a much better chance of them joining the conference than Kansas St or Iowa St. Those 2 schools will never be considered for membership.
UNLV actually would have a shot at the Big 12 if they stay together and expand. The league has already had talks about holding their title game in the new Raiders (and UNLV) Stadium. The results haven’t shown on the field yet, but UNLV athletics has had some large donations from the Fertitta brothers and a few others with the eventual hope that the program can make a big leap once the stadium is ready.