the Big Ten and Big 12 trade Nebraska for West Virginia and then add just BYU and Boise State to put them at 12, no Colorado.
Divisions could be:
BYU, Boise State, Nebraska, Kansas, Kansas State, Iowa State
Oklahoma, Oklahoma State Texas, Texas Tech, Baylor, TCU
Maybe they try to split up a little more evenly, but there would be plenty of years that top division would still compete with that bottom division.