The Atlantic XVIII Conference would be comprised of the teams that new, combined entity wanted.
If that were to actually happen, the Big Ten and SEC would need to even out, so either the SEC poaches 2 from the XII/ACC or they both go to 20. Let's just say 18.
If the SEC had unfettered access to any 2 programs, I THINK they'd do the once-traditional footprint expansion with UNC and UVA. If UNC wouldn't do it without another school from NC, that would complicate things. Let's just say NCST and VT to simplify things.
The new, combined conference would simply draft teams until they get to 18.
There would be enough backroom deals and big-monied handshakes during this time to make the old SWC blush.
The point is, you'd have 3 conferences of 18 each. These 3 champs would host 1st round games in an 8-team playoff.
The G5 would have their own playoff, funded by the big-boy conferences.
54 teams on an even footing would be great. So would 60. Hell, with 60 (20 ea), you could split back into divisions, have everyone play everyone else in their 10-team divisions, and all 6 of them could be in the playoff.
There's even, fair, square ways to do this that seem utterly foreign to the decision-makers.