It sure seems that where this is headed is FCS FB for UTRGV.
The SLC is a natural home for them. Once they have FCS football, it could well become a reality.
It's going to take time and money to get the program up and running, and more time and money to get competitive.
Don't see a FB future for the WAC at this point in time.
It's WAY too early to talk about FBS football at UTRGV, when they haven't even announced a FB program. That's a much larger investment, and the Texas legislature may have something to say about that, when the time comes.
South Alabama, Georgia St, Old Dominion, Charlotte, and UTSA all brought incredibly young football programs into FBS with very short stays in FBS. About a decade ago FAU and FIU did the same thing. I surmise that if UTRGV is super ambitious and has the enrollment and local draw to back it they too could make the giant leap. I would imagine that they would fancy themselves an upstart rival to fellow UT school UTSA.
Honestly, they would fit in well with either conference--Sunbelt or Southland. It's just a matter of where they feel comfortable and who is willing to take them.
With a football program they would fit in nicely as a 14th all sports member and 12th football member of the Southland. 14 is awfully crowded for a 1-bid league though. I'd try to see if UALR and UT-A would be interested in joining and then forming a two 8-team conferences with a scheduling alliance and then try to grow the two leagues from there. It would give everyone better access to the NCAA men's bb tournament and other tournament sports.