I'm seriously thinking about doing just that, and forgo leveling my main job. I'm not really sure how the market will react once Heavensward is up. Supply and demand should both be very high.
Also, Dragoon 48. Gosh I forgot how FATE farming can be mind-numbing. After this one I'll need to level up Warrior (22) and Monk (21), though it may be a better idea to save the bonus exp for the rest... Decisions, decisions...
Oh if your goal is making money, I fully expect this to be an easy and guaranteed way to do so. You have to consider that on every server there's like 1 to a dozen of crafters that are going into the expansion with 100-500+millions with the sole goal of being the first to sell whatever can be crafted/do the airship stuff and so on.
While they might have FC people supplying them materials, it's not necesarily the case(mostly because any sort of investment in materials will be easily covered by the profits from selling their products anyway).
So I expect the prices of the first common/semi rare materials to be insanely high, because the supply will probably be low(most people I'm sure will level their combat classes first) and the demand won't be terribly high but will be from people with insanely deep pockets. So it'll be really easy to make money.
Now wether or not that's fun to you, that's a different question. As someone who has enough money to probably never have to farm any for the next 2-3expansions and still afford anything I'd want, I don't care, but if you've been poor all of 2.0 and you're looking to be rich all of Heavensward, I think banking on this and spending the first couple of weeks just making money is a pretty solid idea. Unless you're in a static and need to start farming alexander normal asap and what not, there's also no real rush to get to cap.