I would doubt T2 and T3 get into the "big boy" playoffs. Each would have their own probably. It would look a lot like UK Soccer with a Premier League, then EFL Champ, EFL 1, EFL2, etc. Just without relegation.
The top 20 are pretty easy, as you say:
OSU,MICH,BAMA,ND,LSU,GA,PSU,TEX,AUB,OK,FLA,CLEM,TAMU,TENN,FSU,USC,WASH,ORE,UCLA,MIA
(Note that all but 4 of these are now consolidated into the B1G and SEC - what a coincidence!)
I think 20 is actually about the right number for media slots and they would stop right there, rather than pull in more. There's only 7 or 8 really prime timeslots on the networks as it is. So 10 games a week (8 or 9 with byes) might be exactly what they want. Plus look at the potential next 4 teams listed below and tell me they belong with the above - not really.
If they did go to 24 or more, then probably it would come down to brand, population/tv market, recent performance, and maybe geography. Those are super-arguable. The next 15 to pick from are probably:
Wisky, Neb, MSU, Arkansas, VT, Iowa, Utah/BYU, Ole Miss, SCar, Texas Tech, Ok St, UNC, Mizzou, Minny.
Hard to pick the best 4 or 8 from that list - not a lot of big differentiators. Another reason I think they'd stop at 20.
All that said... if they took those 20 teams out, and left the other 50ish plus the G5s to be just "college football" again - I would love that league and would happily follow it and not the Premier.