If Oklahoma (and/or Oklahoma State/Kansas) got offers to leave the Big 12, I can definitely envision Texas leaving the Big 12 (at minimum for football) just to try and preserve its control over scheduling quality opponents. At that point, for self-preservation, they would (should) definitely consider an ND-deal with any power conference it chooses. I think the SEC would be a nice fit for them (today, not by 1990's standards), especially if they can bring along a TCU (which would cement the Dallas market for the SEC). They would rejoin A&M, and be well-situated in the top football conference in the country. The drop-off of losing Oklahoma/Kansas/Oklahoma State (in addition to Nebraska, Colorado, A&M and Missouri), IMO, would be too much. A back-fill of any combination of Houston/SMU/CSU/BYU/etc. doesn't regain that lost value. If they wanted to create a Texas-based non-football league, for the sake of travel, I could see that (Texas, Texas Tech, Baylor, TCU, Houston, SMU, Rice, North Texas, UTSA, Texas State), but that is highly unlikely as well.

Even if under different conference banners, Texas and Oklahoma can maintain their rivalry game in OOC play.

The key factor at play is that both Texas and Oklahoma provide immense value to the other, despite "tensions' with the LHN. They also have political factors at play, where they do not wish to see Oklahoma State, Texas Tech, Baylor or TCU "left behind" ala UConn, USF or Cincinnati were in 2013. There are solutions that can create a true P4, without leaving a majority of the B12 out, and ensuring a safe landing spot for the others. It just requires the P4 to, essentially, work together in order to ensure that to happen. Stranger things have happened.