Id say Houston for one, D1 school in a huge city, giving us a firm conference footprint in that metro area we currently lack to a certain extent (and just more leverage for the TV deal).
Houston adds noting to the viewing audience for the Big 12 - last time I checked you already had Texas - which you will probably find more Texas fans in Houston than Houston fans (guessing here - but my experience in big cities is that top U in the state has the fans with the local school just sort of oh aint they cute type thing)
I honestly still see the Big 12 completely imploding sooner than later. Forget the entire sports angle - Texas is one of the few schools in the nation that could both bring something to the CIC and benefit from being a part of the CIC... which is why I still think somehow they end up in the Big Ten.