In light of Big 12 commissioner Bob Bowslby essentially saying today that the conference needs to look at adding teams to become eligible to have a conference championship game, it seems relevant to discuss likely candidates. It's fairly obvious their lack of championship game cost them a spot in the playoff. I don't see many strong choices out there unless they manage to pull a team or 2 from a Power 5 conference, which seems unlikely.
Boise State (Mountain West)
Colorado State (Mountain West)
Southern Miss (Conf USA)
EDIT: Added Cincy and Memphis to likely candidates.