Tbh, old hw drk was not more complicated with its gameplay loop at a base level than nearly any job we have today. Its weird parts were always its tanking side (dark mind requiring darkside+da for example)
Let us begin with drk's combos, the core of any job's gameplay loop is what do you do with your gcds. In this drk in HW had 3 combos
Hard-spinning-power for enmity
Hard-syphon-souleater for recovery
Hard-syphon-delirium for damage
It also had 2 other gcds in the form of bloodspiller and scourge in previous iterations. While bloodspiller has only ever been seen as a inner beast clone, in sb it was seemingly fine. Scourge on the other hand was beloved and its removal was met with uproar.
In aoe, drk had 5 gcds. Unmend, unleash, stalwart soul, abyssal drain and quietus. In HW and SB unmend, unleash and abyssal drain all cost mp. Quietus is a decimate clone.
You also had its primary resource of mp to fuel its core mechanic dark arts. As the dev you know that the removal of dark arts has not gone over well with the drk playerbase so this needs to come back in some form. Previously in HW you could use 4 dark arts before mp was gone, in sb it was nerfed to 3.
Mp costs being removed from unmend and the aoe combo made the job more mp efficient in dungeons. The key is to make the mp recovery strong enough to get back from a death but no so high to spam dark arts or its ogcd attacks edge/flood and dark passenger
In sb, another resource was added in the form of blackblood. This was never very popular, often referred to as inner beast gauge for its identical function. It was mandated by higher ups so it has to exist in some basic form even if its as simple as 1 action like sch.
You also want to try and avoid mt/ot situations and the spam that was SB. So if you were to bring back dark arts, the changed effects need to incentivise swapping between each combo.
To this end I would bring 3 combos back with each finisher being different:
Souleater remains as is in function
Scourge is added to a combo step, Delirium would come after it in the combo and inflict a debuff that recovers mp when the enemy is struck
Spinning-power slash gives the most blackblood.
Dark arts costs 2400mp and can change the effect of these combo finishers
Souleater adds a buff that improves the damage drk deals by 10% for X seconds
Delirium gives a buff "lifesteal ready"
Power slash inflicts a debuff that increases the hp restored by soul eater, bloodspiller or abyssal drain when used on this enemy
Blackblood then becomes a resource for your party buffs:
Soul survivor puts a shield on drk and another victim on the enemy that increases the crit/dhit the enemy takes by 10% for 15s. Costs blackblood
Blood price puts a party wide shield. On breakage this restores 1200 mp and a regen equal to X potency for 15s. Costs blackblood
Other buffs
Blood weapon gives haste for 5 attacks. stack lasts for 30s
Darkside allows execution of Delirium/Souleater/Power slash without breaking or meeting combo requirements for 4 attacks.
Turmoil grants 50 blackblood and a heal. Used ooc for 100 blackblood. Uses SB delirium animation.
Oblation grants lifesteal ready and a free dark arts
Lifesteal ready makes bloodspiller and abyssal drain available. Both deal damage with significant lifesteal, 1 aoe, 1 st.
While I do miss having a counter attack, that would never come back because of parsers :/
The core loop boils down to keeping your buff dot and debuff up and swapping between each combo. From time to time, you can even break combo without consequence to keep another effect up.