That is the perfect scenario for the Big and SEC. But that won't be this go-round.
This realignment could go as follows:
I believe it is likely that the PAC-12 will be very slow to expand since there are no sensible geographic fits with the B12 "leftover" eight. The PAC12 will at first play defense to blunt any try for the B12 to grab the Mountain Time Zone teams: CO, UT AZ and ASU.
There may be some movement by the BiG to "keep up" with the SEC at 16 but their options are Kansas and nothing else - sorry WVU. They very well could go for another contiguous option and pull UVA. Missou isn't going to leave the mega bucks of the SEC to join a conference that they should be in! I don't see the BiG going into Texas/Oklahoma with the available schools.
The ACC is relatively safe at 14+ND 5 games per year. Maybe/long shot WVU could be a "shotgun marriage" (they did let in Louisville.) ND will not cave and join football full-time with the ACC. This will continue to be a limitation in filling-up that conference to 16 members.
There are 65 Power teams today. 16X4 plus ND would work and nobody would be without. But it appears that in the end there could be one bridesmaid - I believe that's Iowa State.
If the non-SEC conferences are slow to add-on and want to stay status quo, The B12 expands to 12 - unlikely to get the Pac12 four schools above - and then they raid the American/Mtn West. Candidates would be from Cinci, Memphis, UCF, USF, Houston, Boise (fb only) and BYU (fb only.) - No to SMU, Tulsa, NexMex, ColSt and Wyoming. The Big 12ers will do the picking not the AAC folks.
Then the issue will be: can the B12 survive with a reasonable TV contract and Power Conference status - We can't survive the Big East-like downgrade to "Group of 5/6" and maintain our athletic budgets. Also, this downgrade would rob TCU of any hope of regular national playoffs spots (assuming 12 slots annually.)