I have yet to see the A321 NEO's projected range (the 320 NEO claims 3,800 NM's of range, 500 extra over the 'standard' model). It could replace the 75, but, if it did, it'd have to be seriously over-powered for the majority of the routes. Unfortunately, that was the downfall of the 757 - it is too good, and burns too much fuel doing it.
According to Airbus, EIKY-KMYR may be within the range of the A319 (the current model) or the 320 NEO, though it's tight (does their range include reserves and alternate fuel?). That's assuming 124 or 150 seats, respectively, and, of course, you'd need 120 min ETOPS. Spirit's got an interesting business model, but I think we all agree this route isn't likely part of it (to charge the fares that Spirit does, you'd need to get more folks onboard to cover the fuel of transoceanic routes and still eek out a profit, so a 330-300 or future 350-1000 would likely be the best option).
The pending battle b/t American, JetBlue, and Spirit from FLL/MIA to Bogota (and points beyond, in Spirit's case) is shaping up to be really interesting, though. Legacy with all the perks, lower cost with TV's and some leg room, or 'meat in the seat' - who will attract the most customers, and who can make a profit at their price point? When Southwest starts going to San Juan is when the sparks will really start to fly!
I thought Spirit didn't have any international layovers, except Lima. If Spirit's doing service
through Bogota, then it sounds like more South American layovers are in the works, or some fancy 3 pilot crew ops, which could be interesting, given the 'rest seat' possibilities on Spirit aircraft.