I have a feeling that "advanced" jobs will actually come into play much earlier in the game. Maybe you'll learn advanced skills as you level your main. So a Lv 20 GLD can be a lv 20 PLD. Maybe there is more than 1 specialization to learn per class. a GLC can spec. PLD or DRK

I do hope they take a cue from FF:Tactics and new jobs become unlocked as you level basic jobs to a certain point.