I don't see that Memphis, Houston, Colorado State, or Cincinnati really bring much to the Big 12. Houston's a big market, but it's not a gain because most of those TVs are likely already watching Big 12 football. They're undermined further by the fact that Texas, Baylor, Texas Tech, and TCU probably don't want another Texas school to split the market.
Add to that, all of those programs are the poor 'little brother' in their own markets, and that's not really all that likely to change with a move up to the Big 12. And, none of them are exactly dominant as mid-majors, so they're not likely to be competitive if they get promoted.
If I was the Big 12 commissioner, and was really intent on expansion, I'd be much more interested in the Arizona schools. They make sense regionally, they're already P-5 programs, and their current TV deal is a big disappointment. Neither of them brings a big market, but they're better than grabbing a couple of mid-majors.
Of course, Texas is the real hurdle. They're not giving up the Longhorn network unless a new deal pays them more money. Not too many programs are going to be willing to join (probably no existing P-5 programs) while Texas has its own network and its own revenue stream. And no network is going to give the Big 12 a big contract if they don't get rights to Texas.