If UT and OK leave the Big12, it's finished. ACC, Pac12, Big10 will take some of the leftovers to get to 16 to mirror the SEC (unless they go bigger then everyone else follows suit).
With the 12 team playoff, I don't see the incentive for ND to join a conference unless the conferences start playing 9-10 conference games leaving ND struggling to fill a legitimate schedule. The goal for most teams is to get to 10-2, 9-3 and hope to sneak in the playoff. Why bother scheduling a difficult OOC game when you'll theoretically have a good schedule already?
I keep going back and forth as to what the ACC should do if they can't get ND. WVU and Ok State makes sense as they wont have a home and they are respectable P5 teams. If we go to 20, pick up Houston and Cincy as they'll be competitive in the ACC and get you into new markets. Hard no on USF & UCF. They bring no value from a football or market perspective.