Adding BYU and AFA would not make any financial sense. The existing B12 schools would certainly take a revenue hit by doing so.
AFA's academic restrictions make it very difficult to compete in the B12. They've had their moments in FB and BB while in the MWC but those would be very rare in the B12. Also, their attendence in FB and BB is not up to B12 standards.
And as long as the B12 isn't desperate to add schools and academics are a factor, Memphis will likely never be an option for the B12.
Sad to say, but their attendance rivals ISU's attendance. For example, Falcon Stadium seats approx 52,500 people and is usually full for every game due to the student body, AF personnel and past grads. JT seats approx 55,000 people and is only full for big time games such as EIU, Nebby, etc... So, when talking attendance, AFA is really the same as ISU.