The thing about college football in Texas is that there's a stratified class system going on in every circle of the sport all the way down to Division 2. Texas looks down on A&M, who looks down on Tech, who looks down on Baylor, who looks down on TCU (only because they're in the MWC), who looks down on the C-USA castaways (UH, Rice, SMU, UTEP), who look down on North Texas, who looks down on Texas State and UTSA, who look down on Sam Houston State, who looks down on Lamar, who looks down on Stephen F. Austin, who looks down on Abilene Christian and Tarleton State. However, it goes beyond just rivalries...NOBODY wants anyone to move up to a better conference except for the institutions themselves, and people will fight tooth and nail to keep their "inferiors" off of their turf. The only way to change those perceptions is if there's a shitload of money in it for the "higher" team. It's this way across the country (hello, TheShocker), the only reason Texas is different is because there's a lot more teams at the 1-a and 1-aa levels jockeying for position.
So basically, unless there's a significant money boost in it for Texas (and A&M to a MUCH lesser extent), TCU won't be invited. About the only way I see them getting an invite is for BYU to get an offer and accept it, and then Beebe Texas decides the Dr. Pepper should be revived.