The Southland believes it could potentially get Texas Southern & Prairie View A&M out of the SWAC. Whether that is realistic, remains to be seen. If those schools didn't make the move when the Texas schools (SHSU, SFA, ACU & Lamar) were in the league, why would they move now? Doesn't make sense, but they are trying. Outside of that, not sure any other SWAC schools are viable additions and likely to join the league.
So you are correct, the options are likely Gulf South programs (West Georgia and West Florida have been mentioned, but likely targets for ASUN > SLC) or Lonestar Conference programs (Commerce, Kingsville) though chances are that the WAC will get the pick of the litter before the SLC if they so desire to go that route for their next football program (if they are unable to add a current Div I institution, which also seems to be their preference if possible).