The problem with giving dark knight a two-handed weapon ability is that too many of the abilities they'll carry over from gladiator are shield-based. I think it's most likely that we see dark knight as a tank, but functioning a bit differently than paladin. Where paladin is far and away the best single-target, damage-mitigating tank, I can see dark knight functioning more similarly to warrior's role in 1.x as a damage-dealing tank that's good at tanking multiple-smaller targets.

I think it would be interesting to see dark knight as an off-tank damager dealer with an ability that allows it to deal more damage as it takes more damage. Perhaps it gets an area of effect darkness-based sword skill? It'll be interesting, and I'm certainly open to new interpretations of jobs similar to what they've done with scholar.