No program is even considered unless they average at least 30k/game over five years in attendance. And no school is considered if they don't have a five year rolling Sagarin average in the top half of FBS (the lowest score is dropped to allow for the occasional down year). You can't claim to be a big time conference if you have bad teams playing in empty stadiums on a consistent basis.

Teams that meet the attendance metric:
Army (32k), Boise State (33k), BYU (58k), Cincinnati (32k), East Carolina (42k), Fresno State (30k), Houston (34k), Memphis (38k), Navy (33k), SDSU (35k), USF (32k), Temple (31k), UCF (35k)
Teams that are getting close: Air Force (28k), Colorado State (28k), UConn (26k), Marshall (25k), Southern Miss (25k), Hawaii (25K)

Teams that meet the performance metric:
Air Force, Appalachian State, Boise State, BYU, Houston, Louisiana Tech, Marshall, Memphis, Navy, SDSU, USF, Temple, Toledo, UCF, Western Kentucky, Western Michigan
Teams that are close: Cincinnati, Fresno State, Utah State

So, going strictly by these two metrics, this would be your conference if the league started in 2019:

BYU
Boise State
San Diego State
Houston
Memphis
Navy
Temple
USF
UCF

Programs that are close: Cincinnati, Fresno State, Air Force