Clearly, Frontier has it's eyes on south of the border. They've said they want to be the airline of choice for Mexican flights from Denver.
But what next? Will they add some inland cities - Monterrey or Mexico City? Or will they stick to resorts? Will they go further south, to Belize perhaps? I believe the A319/318 could go as far as Caracas, Venezuela from Denver.
Then there's the question of overwater aircraft. Presently, Frontier's flight to Cancun hugs the coast. Obviously, it would be a tad quicker and cheaper if they flew direct, but this would require some planes in the fleet to be fitted with life rafts, life vests, etc.
But only having a couple of planes in the fleet for overwater flights doesn't make a lot of sense for one destination, so IF
that ever happens, then they could be looking at some Caribbean routes, such as Denver to Jamaica or Santo Domingo.