I've seen quite a few quests about people wanting a warrior of darkness rather than a warrior of light pointing out in other FF games that a warrior of darkness has infact been the hero bringing balance.

I think there will be a touch of that with Dark Knight. Taking the role in context of Ishgard we see a situation where the holy and ordered structure is the tyrant. A very 'tyranny of the light' thing. Honestly I think this is a better place to take the DK. Lets be honest, they aren't going to let players play bad guys and even borderline evil doesn't fit the narrative.