Flat, you make some good points.
The separation of TAMU & Texas into different leagues is the hard reality that others will probably cite as an example if, for example, KU and OU decide to bolt.
I don't see Texas coming to the PAC. They see themselves (rightly or wrongly) as another Notre Dame, willing and able to go as an independent. You would have to swim too far and too hard upstream to make Texas & the PAC work. That said, there are other possibilities.
TT is essentially a free agent with TAMU's departure. They can do what ever makes sense for them. If OU and OSU go together, the state government would probably be willing to write some big checks to make it happen. The same is probably true for KU and KSU. Of course, your logic of just picking up OU and KU because you only need one from each state in order to tap the media market is true, and that may make one from each state make sense. You mention TCU. There are pro's and con's, but I don't see the PAC ever picking up a conservative religious campus as a member. However, you could tap the Texas market with TT, and that strikes me as being pretty realistic. If the goal is just TV/media markets, KU, OU and TT would deliver 3 states. Any expansion would not be 15 teams; there would need to be one more. Lots of possibilities...again, if you are just trying to expand the media footprint and want a state school, ISU is worth considering. That would bring in the entire old Big 8 footprint and make for 4 logical schools to group together. The same could work if you subbed in OSU and/or KSU for OU and/or KU. Sure, there is a higher national profile with KU and OU, but if the goal was simply media markets, either will do.
I don't see the PAC expanding without at least a fig leaf toward academics. None of the schools mentioned above are bad, but some are definitely better than others from a research funding and academic profile standpoint. And I can't see a 14 team league being realistic; you would almost have to go to 16. If that happened, I see CO, UT, and the Arizonas joining the midwest group.
I'm not sure that expansion is inevitable, but if it happens, I suspect that it will generally resemble what I've noted above.