They have several good schools to add other than going coast to coast. Possible selection right in their on backyard are: Houston, SMU, BYU, Nevada, Air force, and maybe even Boise St. Their biggest problem is finding schools willing to give up their rights!
Their are many fundamental problems with the big dumpster fire conference, but the biggest is that no one really cares about it outside of Texas. Today's conferences are all trying to set up conference networks, but the Big 12-2-2+2 won't be able to do that for at least another 18 years because ESPN and the LHN own the third tier rights for Texas. That will always set them apart compared to the other conferences becasue it's an additional revenue stream that they will be unable to obtain. They can find schools that will give up their rights all day, but until the egomaniacs in Austin sign over there 3rd tier rights, a conference network will never be feasable. Also, while they do have a decent tv deal in terms of money, the tv deal is not that great in terms of exposure and what I mean by that is if Clemson was in the big 12, you would probably see them on tv 1/3 less than you do with the ACC, and that's not even counting the times that the commish rolls over and lets that game be aired on the LHN, which is trying to broadcast 4 games this year.