Here is my "best case scenario":
Pac-11 next adds: Texas, Oklahoma, and is forced to accept Texas Tech and Oklahoma State.
Texas A&M is offered the 16th spot but they build interest with the SEC to expand to 16. SEC gets A&M and Missouri, making LSU-A&M and Mizzou-Arky natural rivals. They then enter "second stage" expansion possibilities depending upon how Big Ten-ACC-Big East dominos fall with targets like Florida State, West Virginia, Louisville, Georgia Tech, and Clemson as possibilities.
The Pac-15 then has to decide, as Buffnik posted in other thread, if they are going with a rotating "pod" system, which would favor Utah being paired with CU, UofA, and ASU. If a simple 8 team division format is used, then I would think Kansas is the favorite. Either of these teams then becomes CU's scheduled "rival" (and this matters because the Pac-10 currently has rivals split gate receipts for their game each year).
Kansas State, Iowa State, and Baylor then search for homes depending upon the fallout of the Big East and Conference USA membership. Now that the MWC is still intact and a probable BCS conference, I believe K-State is their top target and the feeling is mutual. Then Baylor is the next best option as a partner for TCU and to get to 12 teams. The next option being to go "super" with adding Fresno State, Hawaii, Iowa State, and then deciding between UTEP, SMU, or Houston. Most likely Houston gets the nod out of that group and the MWC is now a 16-team superconference that sits squarely in the Pac-16's shadow.
If Cincinnati, Louisiville, Memphis, and Houston are all left twisting in the wind then I could see K-State, Iowa State, and Baylor partnering up with those schools and creating a "Central" conference that would be respectable in most sports, with adding some other teams in and around that footprint.