I am afraid things like gunblades, gunpunches and gunshields will come out not faster than 6.0 or 5.0 when MSQ plot will start to get sexy around Garlemald conquering.
But i think it is possible, they would be actually first exclusive classes for FFXIV., since Those are actually almost fully designed classes already. Not just empty pics, they were in battle. in mechanics.

Let's be honest. How many more classes there is in FF series, that they yet didn't implement, and woudn't be forcely implemented, cause lack of ideas and templates? 2 or 3? (don't gimme bullshit around mime or time mage, those have to be unique classes, that do not have even part of skills already existing in our classes we already have!)

They will have to add those GunThings our villains were using to pull back some players in 3 or 4 years, when FF XXVIII will break the system.

Until they want to redesign whole FFXIV battle system, allowing for more modifications in personal idea of class gameplay.
Then new classes and higher level tiers will be unnesseccery.