Its most likely gonna be a scouting and/or phys ranged job. They could also bite the bullet and put BLU to expansion lvl cap and introduce another limited job in its place.

Personally I would like to see a dual wielding pistol job as the next scouting job ala Dante from DMC. Doing flips and dips while looking stylish af. Instead of mudras you get a style rating where if you combine certain gcds at the end you get a unique finisher and it levels up as you maintain and increase the style meter and rating.

As for phys ranged, I would like to see them try again with the concept of a "gunmage" i.e. Sniper. You get a long range rifle and you go prone (phys ley lines) and shoot from a distance. Much like a Sniper you have to pick your spots carefully as you want to stay prone for as long as possible to deal the most damage, similar to how BLM plays. It's gimmick could be combining different ammos for different attacks and effects. Think of Mustadio from the ivalice raids and you have an idea.