I assume that of the remaining Big 12 schools, the B1G as the alpha dog remaining, would opt to add AAU schools Kansas and Iowa State, leaving the PAC 12 to get to 16 by reluctantly accepting Texas Tech, Kansas State, TCU and Baylor.

Kansas and Iowa State would go to the B1G West division, moving Purdue to the B1G East. Most of their required P4 OOC opponents would be cross division games within the B1G.

I would set up the PAC in a zipper arrangement to give the eastern teams access to both Los Angeles and the Bay Area. These would be my divisions, with suggested OOC opponents in parentheses):

Oregon (Oregon State, Cal, Washington)
Stanford (Cal, Southern Cal, Notre Dame)
Washington State (Washington, Oregon State)
UCLA (Southern Cal, Cal, Washington)
Arizona State (Arizona, Utah)
Colorado (Utah, Arizona)
Kansas State (TCU, Kansas, Missouri)
Texas Tech (Baylor, Texas, Oklahoma State)

TCU (Kansas State, Oklahoma State)
Washington (Washington State, UCLA, Oregon)
Southern Cal (UCLA, Stanford, Notre Dame)
Baylor (Texas Tech, Texas)
Utah (Colorado, Arizona State, BYU)
Arizona (Arizona State, Colorado)
Cal (Stanford, UCLA, Oregon)
Oregon State (Oregon, Washington State)