The Sun Belt would have looked like this (*non football member):
*South Alabama (football member in 2013)
Troy (joined in 2005)
^New Mexico State
^Utah State (football only)
^Idaho (football only)
^Since Houston and Tulane were presumed to have moved to the WAC, the real question would be whether Tulsa, SMU, Rice, and UTEP would choose the Sun Belt over the WAC. New Mexico State, Utah State, and Idaho would join the WAC in 2005, although that was the result of schools joining Conference USA.
I could see Tulsa in a football-only arrangement with the rest of their sports in the Missouri Valley (of which Tulsa was a former member) or the Mid-Continent (now the Summit), alongside Oral Roberts, Centenary, and UMKC. UTEP would probably pass. SMU and Rice would be the question marks.
If St. Louis eventually joined another conference, you'd have to figure that either Louisiana Tech, Troy, or South Alabama gets the last spot in Conference USA. Even if USA moves up the start of its football program nearly a decade, UAB and Southern Miss would probably block them. Louisiana Tech would now be on the western edge of the Conference USA footprint, and the Florida schools would probably move to block Troy since Troy recruits