Navy wants a game in Texas each year, so if Houston leaves, the Cougars will be replaced by Rice or UTSA - Rice will have more appeal to SMU, Tulane, and Tulsa than UTSA.
If the U_F schools go to the Big 12, the American will call up the F_U schools. Cincinnati, if they're still around, among others, will demand a presence in Florida.
UConn is perhaps the trickiest one to replace - does the American exit New England entirely, or does it add UMass? A large part of it may come down to how teams in an 11-team conference travel to Temple. UMass may want to see if New Mexico State can assume the bulk of its 2018 schedule if it does get an invitation.
In the unlikely event both UConn and Temple leave, UMass is out of luck. That would probably happen in conjunction with Cincinnati and one other school leaving, so it could open the doors for Western Kentucky and Middle Tennessee - who could both be in the East with UConn, Temple, and Cincinnati gone.
Another particular logjam may be if only Cincinnati and Memphis get invitations - leaving one hole each in the East and West divisions - if no MAC schools jump, the American may have to add Marshall, Old Dominion, or UMass to the East, and then UAB, Southern Miss, Rice, UTEP, or UTSA to the West. UTEP may be geographically challenging, but the West Division would basically be a repeat of C-USA West from 2005-2013 with Navy taking the place of Rice.