I assume the Big 12's GOR is solid legally, but there is no way to know for sure. Texas or the other schools could have built various trap doors into the agreement. I don't know any sports writer or blogger that has seen it.
If it is solid, then these are the Pac 12's expansion options:
1) BYU - huge national fan base, good academics, good athletics. Massive pain in the butt to deal with.
2) Boise - great football team, not much else athletically. Minor metro area and small state market. Academically it is a step above community college.
3) Nevada - no fan base despite good program. Questionable it can deliver Nevada market from a television standpoint. Basketball is OK. Football coach is great, but is in his late 60's.
4) New Mexico - only 1.8MM people in the state, and the poorest state west of the Mississippi. Only showing this school as an example of how far off the choices drop this fast.
I think the Pac 12 is happy at 12 and doesn't have good expansion options.