I kind of hope that it is more than Astrologian/Machinist/Dark Knight are both job and class at the same time. At a minimum, I hope...
1. Turn "classes" into "jobs"
2. Conceptually, you now level the weapon/magic skill that each job relies on (ex: Gladiator and Paladin have levels associated with Sword/Shield)
3. A job is now defined by its weapon/magic skill, its cross-classing rules and any additional job abilities
4. Job stones are no longer necessary, job selection via a simple gui mechanic or armory
5. stats are per Job, not per "skill" or "class" as we've been calling them.
This would solve some of the awkward conceptual/mechanical problems with the current system. It would also solve the problem of having multiple jobs use the same weapon/magic skill (ACN/SCH/SMN).
The current "classes" would regain some relevance. For example, a player whose primary Job has been Paladin could allocate stats differently for Gladiator to better suit soloing efforts. Her Paladin stats would remain as is. What makes the Gladiator Job special is its flexibility via cross-class skills and options. Likewise, the Paladin Job is special because it gives up flexibility for a stronger tanking role.
From a design standpoint, this would free them from the pattern of having a class and then a job. They simply add a job either tied to a weapon/magic skill already in the game or a new one all together. It would also help them explain how some Jobs automatically start at level 30. After we reach Ishgard, some how we get crash courses in new magic/weapon skills (guns/turrets, great sword, tarot globe). Because of that, when we choose to play as any of the new Jobs, we are already level 30.
Basically, if they don't use the expansion as an opportunity to polish up the Job/armory system nuances - it will just be awkward in general. That is why we keep seeing players wanting the new Jobs to start at level 1. The game established that first there is a "class" and at some point you unlock a "job" somewhat related to the class. They are both different yet and one and the same. Some classes actually have 2 jobs and yadda yadda yadda... 3 new jobs? No new classes? Start at level 30? With the current status quo, it just doesn't make sense.