For DRK if we stick to the same concept of a job having 3 class abilities and DRK job derived from a new class I'd imagine the two class abilities to attached to DRK would be MRD/THM. To create the DRK class while keeping the scythe weaponry would be quite easy if you play around with the meaning of words, the best class name would be "Reaper" it could end up being a short-range mage character that cast spells through the blade of the scythe in order to absorb their attributes and take it as their own. AKA DRK would play as the hybrid TP/MP using type of class where at least 50% plus of it's TP based moves would require MP, but just like DRK counterpart in 11 they would have a small MP pool.

For combo I imagine, Soul Eater, Shadow Blade and Darkness, for 1 in which they are all moves which also deal damage to the user though when using it as a combo the amount of HP taken is greater in exchange for even more damage. The other combo would most likely be based on the absorb commands however when it comes to said commands just like other classes they can't be given every type of down effect just cause it was in XI. Since DRK is corruption of the mind perhaps the absorbs accessible to DRK would only be INT/MND/PIE. Or rather... instead of absorb abilities "Soul Eater" could have a MND down effect, Shadow Blade, VIT down effect (if MND down connected) and Darkness (Acc down is both MND/VIT connected). A uniqued ability not seen for the DRK class could be added called "Reaper" in which the status ailments that the Reaper/DRK landed on the target are healed and instead those stat - turn into stat + for you.

The trick when considering how the DRK class would work is to remember that you don't want to add redundant abilities, so don't except that whatever the class name will be that they'll learn the elemental magic spells, because THM and CON pretty much already have that covered.

Side=Dex/Evasion, Front=VIT/STR/MND, Back=PIE/INT.

So the Soul Eater, Shadow Blade and Darkness combo could instead be MND/VIT/STR weakness, then to compensate for the fact that they can perform a stat down for every attribute make the other 2 combos just 2 abilities rather then 3 or more. So that's about 6 abilities, and 1 DRK ability, which would still leave 9 abilities and 4 to introduce, well 8 abilities if we include Reaper. From tactics could prolly take Sanguine Blade/Infernal Strike give sanguine a TP/MP cost and Infernal a 1.5k TP cost. That would introduce the aspect of a drain and aspir without actually having to use drain and aspir. 6 more abilities
-Weapon Bash (does dmg and chance to stun, can only be used after a parry)
-Dark Seal <not sure if THM still have a spell like this but for Reaper it would work as followed> (Sacrifice x amount of HP to increase the chance of an attribute down effect landing when attacking, aka it increased the odds of MND down landing when "Soul Eater" is used)
-Last Resort (pretty much like Berserk but the defense down is greater and effect doesn't reset when target is hit)..

DRK side we have Darkness, Nether Void, Blood weapon, Weaken and Dread Spikes.

Order of learning the following may be Darkness, Weaken, Nether Void, Dread Spikes and Blood weapon.

-Darkness would be the AOE Dark attack (with chance at lowering STR)
-Weaken would lower the targets resist to status ailments for a short duration
- Nether Void would increase the attribute down effects when you land then, and in effect would increase the amount you gain when using Reaper
-Dread Spikes would be a self target ability that drains HP from targets when hit by physical attacks.
-Blood Weapon would pretty much be a 15 second Bloodbath where auto target attacks and TP moves restore HP upon landing a hit. The other perk would be damage is increased based on how much HP you have, so if you have 100% HP then you'll be doing more dmg, the first effect acts as a way to maximize the dmg you can deal.

As for Reaper we have
Soul Eater(Dark Based dmg chance at MND down)
Shadow Blade(Dark Based dmg chance at VIT down)
Reaper(Take enfeebles you landed and absorb them into positive effects)
Dex Down attack (Dark Based dmg chance at DEX down)
Eva Down attack(Dark Based dmg chance at EVA down)
Pie Down attack(Dark Based dmg chance at PIE down)
INT Down attack(Dark Based dmg chance at INT down)
Sanguine Blade (attack that absorbs HP) Cost TP/MP to use
Infernal Scythe(attack the absorbs MP) Cost TP to use
Weapon Bash(After parry does dmg with chance at stun)
Dark Seal(Sacrifice HP to increase chance of enfeeble landing)
Corrupt(used only after parry deals dmg and more lower targets Defense)
Last Resort(Like Berserk greater defense down and doesn't reset when hit)

Just missing two abilities, 8 are attacks, 2 are after parry, 3 are abilities,

Blind Rage (HP perp cost , lowers accuracy to increase attack power and speed)
Hex (HP/MP perp cost, At the cost of HP/MP increases chances an enfeebles may land on target)

Not sure what lv the Reaper abilities would be learned at but I think the above would be a decent skillset for them, they would be a Burst DMG class at the sacrifice of their own HP while at the same time being able to offer enfeebling effect. I tried to keep the DRK theme in mind when coming up with the skillset as well as how the battle flow in 14 works. Though I was just having fun with this coming up with it all on the fly XD it sounds good on paper though lol.