If Mizzou really is gone, the Big 12 is done. That puts the SEC at 16 teams (A&M, Mizzou, Clemson, FSU).
I see the Big Ten wanting to keep pace, they'll make another run at Notre Dame and this time might get them. I could also see them making an ACC raid on perhaps Duke, North Carolina and either Maryland or Georgia Tech. Pac-12 makes a power play for OU, OSU, Texas and Texas Tech. Big East responds by grabbing KU, KSU and Iowa State to at least get to 12, maybe grab several more schools from the ACC to get to 16 (or the ACC raids the Big East/Big 12 to get to 16).
The only school I think might truly be left out in this whole shakeup would be Baylor.
No, if you had 4 conferences instead of 5, more than 1 school would be left out.
Current numbers are Pac12, Big12-2, Big 10+2, SEC12, BigEast9 (w/ TCU), ACC12 = 67+ND=68...and that's without BYU, Boise, and other decent schools like UCF, Houston, Nevada, US service academies...one could argue if those last schools are decent, but you'd have to bet ISU is in the same ballpark as those schools.