Coastal Carolina could join the Sun Belt in all sports but football in 2016, and then take its time on the FBS transition.
The SBC could also try to take Houston Baptist and/or Incarnate Word from the WAC; if they're going to travel halfway across the country to play conference matches, they can at least cut out the trips that are two time zones away (Seattle, Bakersfield). The WAC will have 11 members this fall in Men's Soccer, and would have 12 if Chicago State gets its team started like it was to have supposed to this fall. NMSU has also talked about adding the sport, in which case the WAC probably calls the Mountain West or Pac 12 and ask them to take a few affiliates off their hands.
If the MWC wanted more soccer schools....wouldn't they try to grab their own school UNM from CUSA, first?
The MWC has five men's soccer playing schools that play in the PAC (San Diego St), WAC (Air Force, UNLV, San Jose St), and CUSA (New Mexico). The schools are so spread out, so I'm not sure it would benefit them to have the Mt West sponsor men's soccer even if they had a sixth member.