re: If the SEC adds Clemson and FSU, I think that'll be the end of SEC expansion

You don't exclude Clemson just because SC doesn't like them, or Fla State because of Fla.

We're headed for super-conferences, where pods/divisions will resemble historical conferences. Oklahoma and Texas will anchor a Midwest pod, with Mizzou, A&M and Arky being easy fits. You're most of the way to a second conference with that already.
If you can cherry-pick ACC schools, you do the same approach- you go for the big dogs, and the others will follow. You then take some to fill in the rest, and go from there. If those two bring UNC along, you might get someone like Ga Tech and NC State, and you have the heart of the old ACC restored under the SEC flag.

Let the Big 10 figure out what to do with the Pac leftovers. Do they want a Big 10 Pac division? Stanford, Washington, Oregon?