Leaving aside what teams make sense (BYU does if they get ASU/Az, but they'll probably go eastward if they can't get those two), a 13 team conference is a logistical nightmare.
So (again assuming they can get the two Arizona schools which isn't a guarantee), it would either have to be just those two, or those two plus BYU and one of Colorado/Utah/Boise/UNLV...etc