I have been thinking about this topic a bit over the past month or two. I have come to the conclusion that it would be wiser to focus on figuring out how to branch a class into two or three jobs without essentially making two or three identical jobs. The end result is not too dissimilar from what the OP proposes. The difference, however, is that the class/job dynamic would be closer to what it is now.

Using the THM example, if a THM was rebuilt such that it had its old debuff mechanics as well as its newer nuking mechanics, then the jobs would be built such that the soul of the Black Mage allows access to only the nuking mechanics of the THM, while the soul of the ... (Necromancer?) would allow access to only the debuff mechanics of the THM. There would likely be exceptions to this rule, and possibly a few spells or abilities only accessible to THM and/or other classes.

As such, what we have is a class that can play different ways, however the player wishes. From that we have multiple jobs that are unique, well-defined, and focused, designed to hone one aspect of the base class at the cost of another. That's how I feel about classes and jobs. Classes should have an array of play styles, while jobs act as the role-defining mechanic they were originally meant to be.