I'm honestly not sure how you guys are going to pull it off~
From a conceptual level, it's a brilliant idea:
1) Classes are balanced, meant for solo.
2) Jobs specialize your class, meant for party (no longer balanced for solo, or at least no longer care if it's good for solo or not).
If the game was built from the ground up to support this, it would make a lot more sense, and I wouldn't be concerned that it could potentially be a disaster.
A good example of how it could work (if everything stays as is, which I doubt), would be for Conjurer to be able to specialize in either Black Mage or White Mage. CON already has nearly all the tools for BLM or WHM. By specializing, you could buff the aspects that match the job, and weaken those that don't. That is, if CON was set up as BLM, all elemental nukes will be increased in damage, while curative magic would be decreased (certain spells locked out, perhaps - BLM with Rebirth would be ridiculous).
Hopefully when you play as a job, certain "off class" abilities now have their full potency (for example, if CON turned into BLM, having a nerfed Drain because it's a THM ability would be pretty silly).
However, it's been stated (or implied, or whatever) that every class with have one job to begin with, and that BLM and WHM would be part of the initial set. That would seem to indicate that THM would be BLM, and CON would be WHM (since CON recently received some pretty clutch WHM spells). However, how would that even work? CON has all of the elemental nukes. Would THM set up for BLM just randomly acquire all of these spells? It would be pretty difficult to just "move the spells over" from CON to THM, since all of the lore of CON talks about the elements. So, unless we're getting a BLM with no elemental nukes, I have no idea what the plan is (but I'm very curious to find out).
Similarly, CON into BLM or WHM is the only thing that really makes sense in my head, since currently most classes are random amalgamations of classic jobs. I'm not sure what you would do to a Lancer to make it more like a Dragoon, besides add Jump and maybe some kind of wyvern summon. What would you weaken, though? It sounds like that would just make it even better at solo.
Or, for Gladiator, making it into a Paladin... The only requisite would be a certain rank in Gladiator? Then suddenly it would have all these Cure spells when set up for Paladin, even if the player has no ranks in Conjurer? Or you would be able to play PLD, and it would buff your curative magic, but only if you HAD Cure spells you could equip? And, again, this seems like it would just make the job even more amazing for solo than it already is.
Honestly, as long as something happens to where I can play a style similar to BLM from FF11 (able to cast massive damage spells, some debuffing and cures, pretty much made of glass otherwise) I'll be excited (because certainly so far CON is a huge letdown in that respect), but I hope the proposed details of the job system are released sooner rather than later so the development team can take into account player feedback. After having to wait a year or so for combat to be entertaining, it would be a massive letdown for the job system to be unappealing.
And, when I hear people speculate on things like "you'll probably need to rank up Archer in order to play Bard", it honestly makes me a bit nervous. How do we go from shooting arrows to playing harps? They're both stringed instruments with similar shapes, just the harp has more strings? >.> Nor do I have any idea how you expect to get Thief out of this mess, since no one uses daggers besides Gladiators, and they call those "swords" somehow. (Also, archers punching things with auto attack is silly. Same with the potential for red mage not being able to use swords because you probably won't base it off of gladiator. Hopefully part of the plan is to take a step or two back away from how the armoury system is currently laid out.)