I doubt that PLD will be replaced as MT for anything serious. The fact that they use a shield means they take currently ~10% less damage than someone without one, and this will only get better with better shields ; that helps a lot their position as a MT. The other thing is, PLD reason to exist is to be the sturdy shield of the party ; they have an awesome collection of defensive CD that is only bound to get better with the expansion. Even though we do not know much about DRK, they will NOT have as good a CD collectiona s PLD, because then the two of them would be too close in design.
That means that while i could see specific instances where DRK is the best MT (like DRK is the best MT for some rare bosses), in most cases, PLD will probably remain the MT of choice for hard progression content. Note that for easier content, who cares ? Everything will go.
As for OT, currently a warrior offers several interesting things. Thing is, their DPS as OT is nothing particular; the only thing great about their DPS is the fact that when they need to pick up tanking every now and then, they will do much more damage than a PLD while in tank stance, since every 2 minutes they can totally negate their damage reduction. They offer some sweet utility with storm's eye and storm parth, but honestly, it is really easy to see DRK offering different but as valuable utility.
In fact, it is likely that they will not compete on what a war is good at either -- which is DPS while in tank stance, especially when not asked to be in it constantly.
Based on all this, my guess would be this: DRK will be best used as an offtank, like WAR, and will provide higher utility / versatility than a WAR, at the cost of a reduced damage output in tank stance. Both PLD/DRK and PLD/WAR combos will be viable, and the mileage of groups may vary between the two.
