Dark Knight wouldn't be that hard to pull off as a Tank dunno why people are over thinking it.
I didn't put much though process into this but I know someone with more creativity and love for Dark Knight can revise this and make it better.
Dark Knight can branch from MRD and TMH easily and with its own version of Defiance that would revolve more around HP, damage, and enmity were certain actions would grant Astral Fire/Umbral Ice.
For now lets just call the Dark Knights version of Defiance Scarlet Delirium.
Scarlet Delirium: Channels damage taken into enhanced attack and magic attack as well as increasing maximum MP by 25% and enmity.
Effect ends upon reuse. Using certain actions while under the effect of Scarlet Delirium you will gain Soul. Each unit of Soul increases critical hit rate by 2%.
When five units are stacked, you will gain Unholy. All Soul stacks are lost when Scarlet Delirium ends.
Souleater: Delivers an attack with a potency of 300. Will consume 10% of your current HP to inflict additional damage. Can only be executed while under Unholy. All Soul stacks are lost when used.
Dark Seal: All spells are instant cast for the next 20 seconds. Can only be executed while under Unholy. All Soul stacks are lost when used.
Pillar of Fire: Delivers an attack with a potency of 200 to all nearby enemies. Can only be executed while under Unholy. All Soul stacks are lost when used.
Darkness: Instantly grants five units of Soul, changing status to Unholy. Can only be used when Scarlet Delirium is active.
Cross Class abilities will involve Swiftcast, Surecast, Fire 2, Blizzard 2, Virus