unlikely that SWA will join either.
Frontier may give it a whirl again now that PVU is more established, but their first attempt to denver was not even close to successful.
Allegiant will probably stick around and my guess is that David Neeleman's new airline Moxy will have a PVU presence. His goal with moxy is point to point travel to secondary airports, so PVU is a perfect choice for that model.
I can see maybe 3 daily airlines max: Allegiant, Moxy and one other low cost (Frontier maybe)