I think it will be Texas State. Not the best, but pickings are slim. I'd prefer to see NMSU, a truly western school for a western conference. Or even both, for nine FB schools and an eight-game schedule, plus two more baseball schools. They might take another shot at Memphis/Tulane but as a Pac-12 fan I'd rather not go outside of the Pacific or Mountain time zones.
There is a $5 million transition fee to move from FCS to FBS. There is also (IIRC) a seven-year wait before you receive any money from the CFP. There are also millions associated with increased staff, scholarships, support staff, etc., and a two-year transition period when you are not eligible for bowls. NDSU could probably absorb the hit, the other schools, no.
G6 FBS schools do not want anyone moving up. It reduces their already-meager slice of the CFP pie, because you know the SEC, Big Ten, ACC and Big 12 are not taking a hit.