Houston was and is the natural choice for Big 12 stardom. But short of Louisianna, no one does petty politics better than Austin. While we're on the subject, Rice, SMU, were permanent residents in the old SWC longer than Houston and would be natural opponents to join up again. Chasing down teams like WVU to join up are bad fits and almost impossible to work up a rivalry.
Colorado and Nebraska, Missouri were old Big 8 teams that grew weary of Texas justice and skedaddled when they had enough. In any direction, it's hard to see the Big 12 picking up any teams that could come close to challenging at the top. Which shouldn't be the point to start with.