Sparsely Grounded Speculation (you've been given fair warning):
Humor me on this, but I have the feeling the new job system will be a mix between FFIX's ability system, FFT's job-branching frame, FFXI's skilling-up system, and FFXII's licence system.
I see a single basic starting job with access to all of the most basic equipment. By using a particular piece of equipment, your character will grow depending on the 'lean' of that piece. The more you use the equipment, the more you lean towards a class type (Fighter/Nuker/Healer/Support and the sub-groups therein).
For example, using a 1H axe will at first just set you down a fighting path (stats, maybe the odd ability or trait), but then you'll gain access to better axes, which may in turn allow you access to better stats, abilities akin to 'zerk & Rampage, and equipment like plate or hauberks. There may even be pieces that allow you to attain the original weapon's traits at a faster pace. Ultimately, while there wont be any particular job separating one player from another, class distinctions will be as clear as the equipment you wear and the abilities you use most often.
In such a system, you could conceivably be a high level mage type, but work on axes, EVE-style, since you can grab the beginner axe (which is still accessible to your 'job') and work your way up from there. This could be the 'try anything' concept SE has in mind, just instead of building up a completely separate job to try out, every available play style is accessible simultaniously. The only limiter, like EVE & FFXII before it, would be effort and time invested, though they could add more if they wanted.
Jus' throwin' that out there.
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