One of the main problems with this concept of jobs stemming from classes is the fact that jobs will be severely restricted & limited due to the nature of the corresponding class. The definition & intention behind jobs in this system is to serve as a more specialized version of the existing class counterpart. In this case, it's impossible for many of the suggestions to come true. Drk for eg simply can not stem from gladiator which is a defensive/shield/tank based role. Not only does it not make sense, but the abilities will work against the nature of a drk role.
This is why many people want the classes to be altered to be more horizontal. Making them more horizontal, and then basing the jobs off of either exaggerated game play aspects for the classes, and/or basing the jobs off of the classes thematically.

Like having a Thief class that has rogue like abilities (sneak attack, enemy debuffs) and also the iconic gil finder, treasure hunter, and then that classes jobs could be off shoots of that class, so in this case Ninja (game play) and Corsair (thematically).

It's also another reason why so many hate the weapon restrictions, it's just too restrictive to the imagination for creating more classes/jobs.