If Dark Knight is added it will more than likely be a tank class. Currently we have two tanks, two healers, two physical melee DPS, 1 physical ranged DPS, and 2 caster DPS. NIN is a melee DPS class so we'll now have three. They've said that we're getting a new tank and a new healer. I still think that the new healer is going to be chemist as the musketeer's guild has been in the game since 1.0 and we still don't have a class that uses guns. Now that leaves the tank class. I cannot imagine that SE would add another melee DPS class to the expansion after adding NIN to 2.4 before adding a new caster DPS. I'm personally hoping for a caster DPS to go alongside with ninja, and the new tank/healer in the expansion.