The Big12 gets poached by the ACC, B1G, and the SEC. Let me explain why...

1) The assumptions. Let's assume the SEC takes OU/UT, the ACC takes WVU (they already have ND in their strange relationship), and the B1G takes Kansas and Iowa State (AAU schools). This would leave the Big12 with: Baylor, KSU, OSU, TCU, and TTU.
2) The moves: Poach the best from the AAC-Cincy, Houston, Memphis, UCF. Poach the best from the MWC: BSU, and SDSU. Add BYU.

Is this perfect? Nope. Is it better than joining the AAC. YES. The new CFP format makes this possible AND make sense. The new Big12 would be:

West: Baylor, BSU, BYU, Houston, SDSU, TTU
East: Cincy, KSU, Memphis, OSU, TCU, UCF.

Is it "P5"? Who knows, but it is "P5" much more so than the current AAC or the MWC. BYU would be dumb not to accept an invite.