These are the G5 schools who have decent fan bases:
2 year avg (2018/2019) attendance over 25k
Memphis 30,178/38,816
East Carolina 32,908/33,134
Boise State 33,068/32,070
Navy 31,464/31,970
USF 38,517/31,823
Fresno State 31,503/31,552
Army 31,693/30,989
San Diego State 31,439/29,896
Temple 28,470/29,480
Air Force 27,701/27,084
Colorado State 29,504/23,338
They can try to get into a P5, go Indy and try to get an at large, or join together into the 6th FBS conference and get an auto bid.
You could probably be more aggressive and go up to a 30k avg to prevent stragglers pumping up fake attendance numbers. Then you can also cut out CSU and Temple from the list above.
The P5 schools in danger at 30k
Washington State 30,091/28,541
Wake 26,842/26,999
Vandy 28,045/26,288
Duke 26,648/25,811
Houston 29,838/25,518 although they are expected to go up moving from AAC to B12.
Funny that outside of Duke I bet the conference wouldn't mind dropping those teams. Having a 30k limit could help these schools sell tickets. If you are a fan then buy a season ticket and not go if you want your team to stay FBS.
You can get to 5 x 16 pretty easy using the 25k rule. Other schools would be left out since they have no conference.
P12
North- Oregon, Oregon State, Washington, Washington State
South- Arizona, Arizona State, UCLA, USC
East-
Boise State, Colorado,
Colorado State, Utah
West- Cal,
Fresno State,
San Diego State, Stanford
B1G
North- Michigan, Michigan State, Minnesota, Wisconsin
South- Illinois, Indiana, Northwestern, Purdue
East- Maryland, Ohio State, Penn State, Rutgers
West- Iowa,
Iowa State,
Kansas, Nebraska
SEC
North- Kentucky, Missouri, Tennessee, Vanderbilt
South- Alabama, LSU, Miss State, Ole Miss
East- Auburn, Florida, Georgia, South Carolina
West- Arkansas, Oklahoma, Texas, Texas A&M
ACC
North- BC, Cuse, Notre Dame, Pitt
South- Clemson, FSU, Georgia Tech, Miami
East- Duke, NC State, UNC, UVA
West-
Cincinnati, Louisville, Virginia Tech, Wake
B12
West-
Air Force, BYU, Kansas State, Oklahoma State
South- Baylor, Houston, TCU, Texas Tech
East-
ECU,
Memphis, UCF,
USF
North-
Army,
Navy,
Temple, West Virginia
CFP has the 5 conference champs plus 2 at larges.