I dunno about most jobs, but the way I could see dark knight being handled with the current system...

Sword/Shield, primary stats STR/VIT, GLD30 THM15, equipable actions GLD/LNC/THM

Lvl 30 Sanguine Blade(random made up name) Increases Auto-attack/skill potency, makes it so abilities that increase enmity no longer do so, allowing the gladiator based class to deal enough damage to be considered a DPS.
Lvl 35 ??? some sort of damage ability that debuffs the target or applies a DoT
lvl 40 Umbral Shield(random name again) Damage is decreased, while HP and enmity is increased, blocked attacks deal small amounts of damage, letting you use your gladiator tools to offtank if needed.
Lvl 45 ??? AoE damage ability that debuffs or silences the target
Lvl 50 ??? Ultimate move, preferably some sort of Dark Knight signature attack from a previous game like Dark wave/Minus Strike... I would avoid moves that sacrifice(kill you) as then people would use them to grief.

pretty much a DPS mirror of PLD.