Even though advanced classes would give us more 'content' I'd actually rather see something closer to a specialization system that replaces the current classes at a certain rank without actually fully replacing them. Conjurer becomes either a BLM or a WHM depending on the path they choose at a specific rank (lets say rank 30) and they can change between the two only while in town. This forces a party role, while still maintaining the flexibility of the current system. They can even make this deeper by making certain weapons or armor only usable on a certain spec thus making the class almost like 2 classes in one for someone to choose and they get the same depth.

If they were to go the advanced classes route though... They would need to decide if they were going to basically make the current classes disposable in the long term or completely rework them to be a shadow of their current selves. The current crop of classes have abilities all over the place. No defined role and thanks to the ability sub system, little to differentiate from the next class. If they rebalance it and include advanced classes... how does that work with the armory system as it currently stands? Would they add new a class of 'sword' or 'shield' into the game to make someone a Paladin? The arms system as it currently stands can't support advanced classes unless they really change it up. Too many of the classic classes already have their weapons taken.