If nothing else, the Pac-12 can survive based purely on geography, unless the Big Ten gets super ambitious and wants to create a super conference with the Pac-12 teams most palatable to their tastes (sorry “State” schools, presumably). I’ve read that it’s the power conference that generates the least amount of revenue, but I cannot see a conference scooping them up with the travel burden it would place on their non-football sports.
The clear answer is the Big 12. It is by far the newest conference, almost collapsed once before as it is, and has several schools that would most certainly not be picked in a hypothetical power conference fantasy draft: Baylor, TCU, Kansas State, and Iowa State bring nothing to the table and/or have a religious affiliation the other power conferences regard as undesirable. You could probably argue for the inclusion of West Virginia, Oklahoma State, and Texas Tech in the latter group as well, but I think the former could be a decent fit for the ACC via their old Big East ties and the latter two are commonly seen as package deals with their wealthier, bigger siblings and have had intermittent football success so they would not be embarrassments the way certain recent conference additions have been (whisper I’m talking about Rutgers).