There are a lot of ways they can go about the class system, I think sometimes the issue comes down to since these jobs follow the pattern of 3 classes per job you may assume that all future classes will follow this same formula.

But perhaps They'll introduce a job like Warlock for Conjurer which can set any Thaumaturge skill. They're main point of interest will be the Astral side of magic, Wind/Earth/Water in which they would obtain the AM of Tornado, Quake, Flood with focus on two abiltiies that increase the potency of single target dmg, while on the other hand THM would unlock a class called Sorcerer which would grant them all the Conjurer skills while given them more focus on the Umbral line of magic maybe Firaja, Blizzaja and Thundaja, and two other abilities focus on AOE dmg increase at the cost of hp/mp.

Then we have Gladiator with DRK which could be Grants all LNC abilities up to 32 (including Traits) as well as MRD non-exclusive abilities but also grant the first 5 MRD traits.

There are a lot of variables which can be used when developing a class you don't have to think of it as the way it's setup now just because all the classes are set that way now. It's quite possible as things evolve we may find ourselves open to more interesting combinations to play with and more unique playstyles based on the situation.