The classes seem to be quite narrow in their focus - Archer is totally about range, Lancer is about being a bit further away, Pugilist, Marauder, and Gladiator are up close, and seem to either hit hard or balance hate amoung themselves. And a lot of Dark Knight-ish abilities seem to have found a home on other classes, too. Thaumaturge can spend HP to increase magic accuracy, as well as Drain and Absorb Spells. Archer consumes HP to find items on the battlefield. Lancer has an ability that consumes HP to increase damage, one that consumes HP to increase TP, and finally a Blood Weapon-esq drain HP on hit, and so also Marauder, which gets Bloodbath to restore a percentage of HP.
It's pretty clear that a greater degree of self-sufficiency is in place on all of these classes. Everyone seems to have a way, sometimes situational, to restore their HP. Most jobs seem to have a way to use HP, MP, and TP as more than just bars which indicate whether they are alive, whether they can cast, or whether they can weapon skill respectivly. These are resources to balance, and I suspect part of the strategy of battle will involve quickly weighing up whether a particular set of commands will leave you with a deficiency in one or more of these resources (HP/MP/TP), meaning other people have to cover for your lack of damage output for a time.
In this scenario, and I'm only speculating here, something like a Dark Knight, seems less likely. It's principle game was big damage dealing and supporting itself with spells at the expense of it's target. Most of that seems to have already been re-distributed. However, in an interview a long way back, "The number of classes we’ll see at the start of service isn’t set yet. However, they do think they will add classes with version updates rather than with major expansions like they did with FFXI."
If classes are to be added with version updates, I think that supports the notion that they are tightly focused ability-sets, and we already know that the weapon determins the class.