Oh i would kill to play a Dark Knight, ever since FF4 I was in love with that class. BM is my universal favorite.

If they choose an existing two classes then it have to be either Glad and BM or MRD and BM, only options that make kind of since.
I say Glad and BM as kind of the two sides of the same coin that most games present them as, using the same type of weapons and armor. Making another viable Tank Class with some decent damage output.
MRD and BM seem to make since, from what i have heard of the MRD skills and to give another option.

I have not checked out either of the skill sets but i plan to get around to all War/Magic classes and max them. I think going with GLD though to give a third Tank Class for people to choose from since there are already 5 DPS jobs for people to pick between. It is entirely possible though that they could add a new class as a starting class for it, though would raise the question of what to put as that ones second job class.