My theory is that the new classes/jobs will be in the expansion and the expansion will do something similar to what 2.0 did for 1.0; that is to say that when the expansion hits (3.0?), the world of Eorzea will be farther into the future chronologically and the world will slightly change to cope with a thriving large civilization. That'd also be the time where Revenant's Toll becomes a City-State and frontier-type areas are added, hopefully even Sharlayan, Ishgard and Ala Mhigo.
Adding in the new class behind story-mode completion is something I would be alright with up until said first expansion. (Rise of the Ascians, Chains of Bahamut, Treasures of Doma, Wings of the Twelve, Seekers of Sharlayan :-P )
