I see one of two scenarios playing out if CUSA is left with FIU, UTEP, and La Tech:
1) CUSA backfills with whomever they can get. They may try to invite NMSU as a football-only affiliate, at least until the WAC has its FBS legs. I don't see them going after UMass and UConn as football affiliates, but one never knows. These are desperate times for ol' CUSA. Everyone else would have to come from FCS. Whether or not they pursue the current FBS indies, CUSA is left as an incredibly weakened conference for football and probably hoops. It's no wonder Liberty backed out.
2) FIU, UTEP, and La Tech survey the landscape and arrive at the same decision Liberty did: football independence is better than this. UTEP could go to the WAC for OLY sports, so it's an easy transition for them. FIU could ask to rejoin the ASUN as a non-football member. La Tech... (sigh)... La Tech chose poorly. They'll have some options; just not any good ones. They could be a far-eastern member of the WAC, or a far-western member of the ASUN, or a far-southern member of the OVC. I absolutely, positively do not see them joining the Southland in any capacity. This scenario would effectively end CUSA and the G5 would become the G4. And all the FCS schools talking to CUSA about moving up would be left in the desolate wasteland of the championship subdivision for at least a few more years.
EDIT:
Gem's suggestion here opened up a possible 3rd scenario, involving a gentleman's arrangement between the WAC, ASUN, and CUSA. It'd be way more complicated than it sounds, but Gumbart is just psycho enough to make it happen. Not sure if this scenario would actually come to pass, but I wouldn't be surprised if it did.