I think every conference to a certain degree wants to affiliate with like institutions. But the Intermountain West is a different region for college conference alignment. It lacks population like all the other conferences do, however its a premeir conference for a handful of states in which their 9 institutions are located in. Particularly Utah, Nevada, New Mexico, Wyoming and in Colorado too, sharing with the Big 12 that state.)
Not only does the BCS look if you carry the state athletically, but it also looks at if your schools schools carry metro markets.
San Diego St is the only 1-A school in the San Diego market. USC and UCLA carry L.A., while Cal and Stanford carry the Bay area. When they aren't winning, the PAC-10 Californian schools draw in the 40-50k range much like Pitt or Syracuse. An argument for SDSU being first tier is very plausible.
The case for TCU as a first tier market carrier is more questionable, but it does have merit. TCU is the highest profile athletic conference located in the Dallas-FtWorth metroplex. The next nearest BCS school is I believe Oklahoma 3-4 hours away. If Texas was in the greater Dallas-Ft Worth metropolitan area then TC as a first tier would be a much tougher argument to sell.
As to what the next expansion move would be by the MWC, the focus is going to be on obtaining BCS membership. I would have to say Boise St has the best argument for the 10th position in the MWC because of play on the field, and they are the closest resemblance of a major program in a growing state of 1.5 million people. Fresno and UTEP are also candidates with their major support numbers in both football and basketball, but marketwise they can't lay claim to anything more than their metro areas.
Montana isn't far enough along in football yet to be a serious candidate for the MWC. The WAC isn't a very good league to join right now with the instability and travel costs for Montana but they've got to make the investment in 1-A if they want that coveted MWC bid.