Certain Pac 12 and Big 10 member schools essentially join forces to form a competing league with the SEC. Either totally team up (less likely) or form a new league but not everyone would be invited from both conferences. Maybe something like the following:

Prominent members of the Pac 12:

USC
UCLA
Stanford
California
Oregon
Washington
Arizona
Arizona State

Combined with prominent members of the Big 10:

Ohio State
Michigan
Michigan State
Winsonsin
Iowa
Penn State
Indiana
Illinois/Minnesota

Anyone left out of this league would either go to the ACC (on the east coast) or join with some of the rest of the Big 12 remnants.