I can see 4 leagues with 16 teams - it's not that simple though.
I agree, it certainly wouldn't be that simple. There are going to be a fair amount of decent football schools on the outside looking in, and I'm not sure that the BYU's of the world make the cut if they were to continue to play hard ball. Here's how I look at it:
Big10, Pac12, Big12, SEC are the four "controlling" conferences. They currently have 46 combined members. If they were to go to a combined 64 schools (16 each), then that means there are 18 slots left. Without going into who goes where, here are what I believe to be the viable candidates for the final 64:
Big East: UConn, Rutgers, Louisville, Boise, Cincy, USF. (Cincy, USF are on the outside looking in, and I don't think SDSU goes anywhere and UCF is a real stretch also)
ACC: Pitt, Fla St., Clem, Miami, NCState, Ga Tech, UNC, Va Tech, UVA, Maryland, Syracuse, BC (Syracuse, BC on the outside looking in, and I really feel that Duke football will cost Duke a seat at the table in the end despite this little "uptick").
Independent: ND, BYU (BYU could cost themselves, and Navy is well on the outside)
MAC, Sunbelt, C-USA, current WAC, MW:
none
I've listed 20 teams for 18 spots, and that was with the exclusion of Duke, who's olympic sports and academic reputation may be enough to save them. Yikes...