Dont get me wrong dark knight is an amazing DD right now .... but i find the job very one dimensional and it lacks a certain versatility and combinable potential with other jobs.

im going to take this much like my monk rundown and break it down into pieces .... the roles i see dark easily filling are as follows ::

Tank
Damage dealer.
Magic damage dealer.

Lets start off on the first suggestion :: Tank

and much like a lot of other posts i have lets start off with a stance ... this would be a new ability to the game i call the stance ::

Shared Misery(lvl 49 or lower) :: unlike paladin or monk instead of negating negative status ailments the dark knight instead reflects debuffs on it to all active enemies (aura) and is immune to complete disables with a cost ... when the dark knight suffers a stun, paralyze, petrify, death, etc. anything that either instantly kills or completely disables or stops action the dark knight is instead slowed stacking with regular slow for the duration of the effect. Shared misery will lockout any other dark knight stance. as with all stances zoning or refreshing cooldown would reset the opportunity to swap stances(corsair). attack is lost while in this stance defense is gained and magic defense (even spread weaker than pld or run in each but adds to both). this stance will also generate extra enmity with all actions.

Under Shared Misery a few job abilities would work differently ::

(Spell) Endrain :: this spell needs to be added at 49 or lower and be % damage based.
(Spell) Endrain II :: this spell needs to be added at higher than 49 (probably 99) and be a higher percent of damage dealt on hit.
Drain spikes :: will now generate enmity on hit and that enmity is enhanced by +enmity
both of these spells drain percentages would be enhanced by shared misery.
and endark JP abilities will include these spells too.

Essentially dark knight will be a "drain tank"

Last resort will no longer cost defense in this stance but its attack bonus will be dependant on how many debuffs you have on you when you activate it.

Weapon Bash :: will dispel one enemy effect in this stance and weapon bash plus gives resistable extra tries.

Dark Seal:: upon activation also makes the next drain, Drain II, aspir or aspir II spell aoe and generate increased emnity. Drain II's duration lasts until the enemy it drained from is killed, the amount is overwritten by a larger amount, or the regular duration whichever is longer.

Diabolic Eye :: Creates three Shadows if the shadows are struck the dark knight counter attacks with a high critical percentage. these counter attacks generate enmity and give tp.

now to equip this set you will need a set that adds DT- for tanking, and maybe a Scythe that has some DT- II as scythe has the better survivability weapon skills. maybe bend apocalypse's stats a little to make it a tank weapon since its pretty much derided as far as a damage dealing weapon.