As a Longhorn fan I'm torn on this subject but as a MWC fan this seems obvious, Missouri joining the SEC is the best thing to ever happen to the Mountain West. Why?
Well, the Big 12 was contemplating just expanding to 10 after losing A&M, but now they are worried that they will be seen as an inferior conference (despite what the stats say) if they only have 10 members while the other major conferences will have 12 (B1G/PAC), 14 (SEC/ACC), and possibly even 16 (ACC+UConn & Rutgers). Not only are they worried about losing what little prestige they have left, many schools (primarily in what used to be called the Big 12 North) are worried about being left behind by TX/OU and know that there is strength in numbers.
Now many have speculated that the Big 12 may target MW members like Air Force, Boise St, UNLV, SD St, and potential/former members like BYU, TCU, and Houston; however it seems that their gaze has moved Eastward. With the defections of Syracuse and Pittsburgh to the ACC, the Big East is ripe for the picking, and although many on here (including myself) beleive that many of the MW schools are better than the Big East schools the perception of adding current AQ's into the Big 12 will strengthen its reputation more than adding non-AQs.
This means that schools like Louisville, Cincinnati, and West Virginia are going to be the most probable additions to get back to 12. This makes sense from Kansas and company's perspective, why travel West when their are closer options available to them, and IF the PAC16 with the TX/OK schools was to form would the Big 12 would still be in a strong position to invite many schools like Temple, Memphis and East Carolina into the league to keep it viable.
With Big East football all but gone, their AQ would most probably be given to the MWC.
This would be the best scenario for the MWC, they could keep TCU (or add BYU if the Big 12 doesn't), and have the ability to poach the best available non-AQ schools without fear of losing their AQ status, so adding Utah St and San Jose St (for their markets) and UTEP, SMU, Houston, and Gonzaga (as a non-football school) would make a solid all around league, while adding 4-5 new markets. This 16 team league would solidify the MWC's position in the top 6 by knocked CUSA down a couple of pegs (thus eliminating their biggest threat, besides any BCS reforms) and would probably kill the WAC at the same time making them the only other conference in the West along with the PAC12, which would hopefully give them the ability to finally renegotiate their TV contract.
CUSA would respond by adding North Texas and Louisiana Tech, in addition to USF (from the Big East), Temple, and possibly Troy; and the Sunbelt would take the remaining teams in the Southwest like Texas State, UTSA, UT-Arlington, NMSU from the WAC.
In the end I can't think of a better scenario than what is unfolding at the moment. If the Big 12's focus stays East and out of Texas (save BYU or TCU) then the Mountain West appears to be the greatest benefactor of this next round of expansion more so than the SEC. The SEC is hoping to renegotiate its TV deals to increase its payout from around 20 million per school, per year to 30 million, however the MWC could conceivably get a deal that increases its payout from less than 2 million per school per year to upwards of 15 million per schools per year (what the Big East and Big 12 projected payout would have been). Now nothing is set in stone but obviously the revenue increase of even just 10 million a year means much more to schools making 1-2 million versus one's making around 20 million. If this 12 member swing plus the addition of an AQ doesn't give them enough to renegotiate their TV contract then I don't know what will.
If the Big 12 loses Mizzou, and adds Lville, Cinncy, WVU, and BYU; then I think the MWC would be insane not to take advantage of its opportunity to expand and take over the land from Texas to California and everything in between.