Well even if they don't add a new class with every job they system and there statements of questing for more abilities, it is possible they can force content completion similar to forcing xping. For instance let say they add red mage to gla and sniper to arc in one update. Because of the addition together they could add long expansive quest lines that require other events such as dungeon completion behest, specific fetch quest, so on and so forth for the stronger abilities and in addition add new abilities to the other jobs that require the same. Its a cheap tactic yes but it allows for more writing and lore addition as well as reasons to seen people into less explored areas.