Make it into a DPS and not a bargain sale Rune Fencer (aka Mystic Knight, Spell Blade, Celes, various other names it's had over the course of the entire series).
The only thing they can do now is give it a ability to allow for it to over flow it's HP, like Drain 3 from 11. Put Drain 3 on a timer. Introduce another stance that drains HP that enhances your damage. But you're going to get people complaining about that.
Other than that there are enfeebles. But it seems their moving slowly away from that or handing it out as a job "role class' option. The way the game is balanced and designed currently, it's impossible for a Dark Knight to actually exist.
Dread Spikes would be seen as OP, but could work as a ultimate, as it would be hollow ground +1 in this game since Resist's don't exist. And that's about it.
So basically, until they learn to balance things or stop caring about making every job apart of the meta (which there will never be perfect job balance), they are never going to be able to make a classic Dark Knight.
Because SE has no idea what they want to do with Dark Knight and can't keep it true to it's true self. Most of the games Dark Knight played within the same theme. Do damage to yourself to enhance your attacks and sometimes enfeeble your enemies to give yourself a upper hand.
Yet Final Fantasy 11 is still going and getting updated as well. Go figure.
While every one had their own "take" on jobs, they all also stuck to a particular theme for the job. Se just happen to take another job and renamed it to Dark Knight, more about that below.
It never worked as a tank. There was no "tank" in traditional JRPGs. Show me a ability that Dark Knight had in any stand alone RPG that it got from it's own skill set that caused the enemy to hit it. It doesn't exist. It can only be classified as a DPS. If front line, armor, and damage reduction (which FF4 DRK didn't have for those saying it did) are your only arguments, by that logic Samurai and Monk are also tanks in 14, due to the fact that Monk has Earth Stance and Riddle of Earth, and Samurai has 3rd eye and then a 20% self heal. The most important thing that a tank has is it's ability to keep the attention of the enemy and abilities to facilitate this function.
There is also the fact that in every iteration except maybe the first appearance of it being playable in 3, Dark Knight has been a job that has focused on giving up it's health to deal damage and enfeebling. Outside of the original version of 3, it has never had anything to reduce the damage to the party or assist in recovery. Paladin on the other hand is easy to make into a tank due to the fact it offered Cover and White Magic. Making it a "tank" as it's the only job that could actually take hits for other characters for years.
The current version of DRK is a direct rip off of Rune Fencer. It even has Rune Fencer abilities, such as Blood Price (named spellblade from it's game tatics advance). The original HW version of DRK pulled even more from Rune Fencer traits as well. Such as wanting it to focus on Parry and Evasion. It's magic reduction abilities are also a Rune Fencer's trait as well. The only thing that has anything to do with a true Dark Knight and has since the start is Soul Eater (which doesn't fucntion the same way, but that's ok), Blood weapon (Which use to function as Blood weapon does now), and Abyssal Drain (functions as another drain, but is the first in the series).
In short Yoshida or his job design team set themselves up for this and bit off more than they can chew. Final Fantasy has been using particular themes for jobs and monsters for years.
Last but not least, those saying that they don't want to look to FF11 Dark Knight as a tank. FF11 is the only other Final Fantasy where it could tank and kept it's identity, as all of it's abilities generated a large amount of Aggro.