Real factor that this hinges on is what classification does SE hinge on when developing new jobs. Could be primary stat, but it could just as easily be basic role (tank/heal/dps) or enhanced role (tank/heal/melee/ranged/caster) or some other classification that is either known to the players or even some internal marker only known among the devs.

We know what's being requested. Red Mage, Blue Mage and Samurai are all jobs requested to be implemented, yet this is also their biggest hurdle as many of these requests come from veterans of XI who will likely complain because these job XIV incarnations will not be the same as what they knew from their "good ol' days". Other requested jobs such as Dancer will have difficulty in application of a support-type concept being placed into the current role trinity, while other jobs like Time Mage likely won't see action in XIV simply due to having applicable concepts from them implemented in other jobs (in the case of Time Mage, see Astrologian).

We know, more or less, the regions that 4.0 will most likely take us and that the story drawn from a particular region can sometimes be reflected in a particular job. Although we already have Monk reflecting one facet of Ala Mhigo, there is still the technological aspect of the former city-state and likely something that would set it apart from the Magitek theme of the Garleans. Included in this would also likely be Xelphatol, a region where not much is known beyond it being filled with canyons and being the homeland of the Ixal. Although we may not reach Garlemald in 4.x, we will likely still reach their doorstep by the end of the expansion.

Lastly, we know that there will most likely be a repeated release pattern of one tank, one healer and one damage dealer. However, this is a more unfamiliar situation in that we might know more about what jobs are coming up than what roles they might be. Although Samurai has been known to XI players as a tank based on some merits now somewhat recognizable in Dark Knight, Samurai could be either a tank or a dps depending on whether the dev team follows the traditional image of the armored shogun held by the playerbase or instead follows Yoshi-P's idea of the lightly-garbed swordmaster. As the jack-of-all-trades of the FF series, Red Mage is even trickier to pin a role on as it could realistically fit any role given to it.

In the end, what do I speculate? Well, a Samurai that would disappoint XI veterans, a Red Mage style that no one would predict, and a freshly made job that incorporates ideas from other requested job concepts.