I think this is a way they could make it work with the base class being GLD:

In regards to weapon, similarly to how a BLM and WHM can wield a one handed weapon and a two-handed, a DRK can wield a sword and shield combo or a two-handed sword. The two-handed sword would be encouraged due to it most likely having more strength and DPS starts over tanking ones. However, if someone found a way to make DRK a viable tank, then true they aren't using it as intended, but it shouldn't be shunned.

The nature of the class from what I recall has always been a self damaging class, in that it sacrifices a part of itself to deal greater damage, in this aspect some skills, or at least one or two in the least, could be high damaging moves but cost HP to use. The DRK could also cost dots on the enemy and if the dots crit, they can restore health. (Similar to an Annihilation Marauder from SWTOR).
With this, it would provide variety and keep players from falling into "rotations" and keep them with a priority system, which in my opinion is much more enjoyable to play. They can cast all their dots and get healed, having more health to sacrifice with so they can use their high damaging attacks.
They can also change Riot Blade to restore HP, but lower it's damage. So in that, you can spend a cooldown or two comboing Riot Blade to restore your health for a bigger damaging hit later on, this would create a need for situational awareness in places like Titan, where you have to consider when the best time is to use HP costing moves so you won't get wiped from things like Tumult.

Defensive cooldowns could easily be changed into offensive ones, adjusted if deemed too powerful. One cooldown could grant an auto-crit on dots so you'd have a change to at least get a %100 or at least even just a %50 on heals so you could have a semi if not fully guaranteed burst period of DPS.

The two Oaths, Spirit's Within and Hallowed Ground are definitely PLD sounding moves, but they are PLD exclusive. If you were inclined to include Oaths, you could have a resting oath and an attacking oath, one where your HP wouldn't regen but you'd get a slight DPS increase or a DOT crit percentage increase, and the other would decrease DPS for a welcome HP regen. This would also incur the player to consider their situation and change between the two mid-battle, which I believe could have been an intention with Sword and Shield Oath but wasn't completely there...

I'm not sure how Spirit's Within could become a DRK like ability, though Hallowed Ground could be replaced with an ability that for 10 or 30 seconds every ability you used generated no aggro, if you were a smart player you'd pop this on the last boss or in a tight spot with convalescence so the healer could keep you up as you absolutely tear through the boss.

Now, for non PLD specific abilities, Rage of Halone would definitely need a name change, as for it's effect it would have to be changed from enmity generation, though this is also pertaining to other abilities too that have the same effect. I suppose the simple thing is would be to put dots on if you were going that route previously mentioned, or just to have the enemy's slashing resistance lowered if combo-ed.

Shield Swipe and Shield Bash are definitely two abilities that would need urgent adjustments due to just the fact a DRK won't be using a shield most likely. These could be replaced with a Sword Bash skill, as I'm certain bashing someone with a Zweihander would have similar effects as bashing them with a shield, though Shield Swipe could just become a sword or grand swipe ability, perhaps the aforementioned hp-costing ability. Though, this is to the developers.

With the fact that the class is based around using it's HP to deal more damage, the VIT traits won't need to change much, as a higher HP is welcome, though not as much as a tank would need as they're not taking majorly large hits like a tank should be.
Circle of Scorn would need to be changed visually, along with it's name, but honestly the effect isn't really a hassle.

Any way, thanks for reading~