My Assumptions:
1) Jobs (or really any additions) are chosen based on what will sell the expansion/get fans excited. $$$ (obvious statement is obvious).
1b) Jobs are also chosen by sheer amount of work involved. The more assets required, the less likely; jobs which can recycle existing assets, more likely.
2) Job choices are not forced to fill missing "gaps" in roles. (4 magical ranged including BLU before a 3rd physical ranged)
3) The way roles are organized is fluid. (2.0 initial release organized Magical and Physical ranged DPS as simply Ranged DPS)
4) New jobs can be influenced by the place we're going, but this is not a requirement. (YoshiP was originally planning to do SAM in HW)
5) YoshiP has repeatedly said he wants new, original ideas (Ravana, Mechanist, Hrothgar race), but there is a strong trend to use pre-existing established jobs.
Job History:
2.0: 2 Tanks, 2 Healers, 2 Melee DPS, 3 Ranged DPS (Initial Release)
2.x:+1 Melee (Balance of 3 Melee and 3 Ranged DPS)
3.0 +1 Tank, +1 Ranged, +1 Healer. (Ranged "split" to Magical Ranged and Physical Ranged)
4.0 +1 Magical Ranged, +1 Melee
4.x +1 Magic Ranged (Can be ignored, it's BLU)
5.0 +1 Physical Ranged, +1 Tank