I was going to say tcu, but the one problem I see is that it puts a second private school in the South.
While Yes, tcu does have good financial backing, until the last 2 games of the regular season, they still weren't filling their football stadium.
However this would re-hash more of the old SWC rivalries for them, and they could keep the SMU intra-Metroplex rivalry on their OOC. Thus giving them a strong slate of home games that would likely sell well, and given the access to Dallas visiting Big 12 teams would likely bring a strong number of fans as well.
And yeah, you're right Arkansas probably doesn't leave the lucrative TV contract fo the SEC, and while it doesn't have massive impact, adding a school located directly in an MSA of 5.5 million people never hurts when your doing a TV contract.