Current trends are suggesting that a FSU, Miami, Clemson, and Georgia Tech will go to the big 12.
Although it may just be talk this article is convinced http://dev.chuckoliver.net/2012/05/fsu-to-the-big-12-it-is-inevitable/
If this happens The SEC will most likely pick up VT and NCst. With these two conferences forming 16 team super conferences the big 10 and ND will be forced to act - meaning the big ten will pick up ND and another leftover team from the ACC (maryland and Uconn would also be a possibilities). The Pac 12 could then be forced to keep up, making them need four more teams. There are not very many post graduate focused universities with established football programs to be eligible candidates. They would have to compromise. The short list would pretty much be the old MWC/WAC
1. Boise St
5. Colorado st
6. Nevada.... point being in this scenario it would be hard to see BYU being left out.
Alternative reality #2
Only two teams, say FLst and Miami (Florida st doesn't want to make the move without Miami) make the move to the big12. The ACC remains relevant inviting two big east teams (say Louisville and Cincy) and expansion dies leaving BYU to play in a crappy Big eat/MWC or remain Indy.
With BCS bowls becoming less relevant, Indy is better than a crappy depleted big east in my opinion
Alternative reality #3
Talks with Flst go south.... This could result in a number of things depending on who you talk to. Big 12 can stay happy at ten members or expand to twelve. Twelve is more likely to happen in my opinion with Louisville getting one spot and BYU fighting it out with Cincy and Uconn for the second.