Hate to say it.. but i doubt there will be a DRK class. Not in the forseeable future anyway. If they do it will most likely be a tank. Being there adding in ninja(Tank), thief(?)(dps) in the Expantion, and the duel pistol(dps) class in 2.2.
The next class they add will probably be a healer, as there already adding in dps and tank classes in the future. I would personally like ninja to be dps, but in FFXI it was a tank class. So.. i'd assume it'll be a tank in ffxiv as well. An evasion/blinding/parry tank.
I would put my money on Blue mage, a status inflicting healer class. Debuffs and buffs to many mobs at once, and full party buffs.