So a Dark Knight walks into a bar...

[Delsus]

Yeah, DRK could spawn from the MRD class like WAR does and get the ability. That would make sense actually.

They would likely spawn from GLA, because DRK and PLD are complete oposites, while PLD is mainly defence and helping the other party members, DRK is more about doing as much damage as possible whatever it costs, also if you look back in FF lore (paticularly FFIV) PLD is a holy knight and DRK is well a Dark knight, if you take away thier respective holy and dark abilities they would be identical, which is why DRK should come from GLA, (I haven't done the PLD story) perhaps they take a different route to PLD, if they were released at the same time, I would say why not have a choice to follow the holy side or the dark side, choose to defend people or choose to be a loner, fighting for noone in the first PLD quest, choose the light you become PLD, choose the dark you become DRK, but that wouldn't really work now.

Also Alerith, Souleater and Darkness are pretty much the same ability, you never see them both in the same game (feel free to prove me wrong) also your abilities (to me anyway) don't seem to show the DRKs idea of dealing as much damage as possible whatever the cost, need something that will inflict a debuff on the DRK to raise attack, or more "sacrifice half your remaining HP to deal damage" kind of thing.