re: If SEC ever expanded from 14 to 16...

Make the assumption that the SEC would prefer to expand into a state where they presently lack a presence (the cable television incentive essentially). Also assume a somewhat rational geographic footprint.

Adjacent states - population:

North Carolina - 9.6 million
Virginia - 8.1 million
Oklahoma - 3.8 million

That makes schools like Va Tech, NC State, Wake Forest, ECU, Virginia, OU, and even Ok State possibilities IMO

I don't see Texas joining but they would obviously be another potential get.

The SEC also touches on states like Illinois (MU and KY for you geography majors) and while that is a monster state (population of 12.9 million) and has a natural rivalry with MU that seems like a pretty unlikely idea overall