Utah vs. CU is the only "rivalry" in the PAC 12 that has no real basis or history. If CU goes home to the Big XII, then just take Houston, too, and call it good.
BYU is far and away the best choice to replace CU in the PAC 12, a fact that has been well-documented and highlighted in the Big XII campaign (if PAC 12 presidents can get over their collective anti-all-that-makes-BYU-different bias).