Considering the marauder/warrior storyline, I am only guessing that Dark Knight will stem from Marauder -- not Gladiator. I don't see it being a tank, but I do see it capable of taking heavy hits (if ever needed). Of course... this is all speculation. We won't know until it's released, blah blah blah.

Quote Originally Posted by dbrewer225 View Post
while we are on the subject DO NOT MAKE NINJA A TANK. that is all.
SE originally intended ninja to be a DPS in FFXI, but it was the community that found ways to 'turn ninja into a tank'. Eventually SE decided to follow the lead of the community, and made ninja more 'tanking friendly'. I could see ninja being the dps counterpart of a samurai tank (two jobs branching from the same class).