Why be arbitrary? Let Notre Dame remain independent, and include the top 80 other schools based on average attendance. All current P5 schools are already in that group. The schools you would add, in order of attendance rank, are:
BYU
ECU
UCF
Fresno St
Navy
USF
Boise
Army
San Diego St
Air Force
Cincy
Hawaii
UConn
UT San Antonio
UTEP
Memphis
The good news in this arrangement is that all three service academies make the cut. The bad news is that somebody has to take Army and Navy. The problem for everybody is that they would have to take teams into their conference that they have already indicated they don't want to play with.
My arrangement:
PAC adds Hawaii, Fresno, San Diego and Boise.
Big 12 adds BYU, Air Force, UTEP, San Antonio, Memphis and Cincinnati.
B1G adds Army and Navy (football only).
SEC adds ECU and USF.
ACC adds UConn and UCF.
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Keep in mind, this is purely hypothetical, and won't happen in a million years.
OK, Houston fans. Let me have it.