This depends on which direction you want to take the cooldown. For Dread Spikes to compete as an Oh Shit button, the effect would have to negate incoming damage, converting it to HP (ex: If you're a DRK main tanking Bahamut and pop Dread Spikes before Ahk Morn, instead of taking damage from each hit, you instead recover HP per hit). If you want another suggestion, here's something I've suggested in the past:
50 Shadow Guardian - Transfer all damage taken for the next 10s to your shadow. Upon expiring, receive accumulated damage over 15s. Accumulated damage cannot be greater than your max HP. Effect expires upon end of duration or if accumulated damage is equal to your max HP. Cooldown: 180s.
Spoiler:
Living Dead is clearly a call out to Fray and his role in the 30-50 DRK quest. Shadow Guardian is instead a call out to your inner darkness, who via this skill would take damage for you for a bit before transferring it to you (because your inner darkness is part of you, after all).
You'd probably have to change how much MP is used for Dark Arts to add another combo, since the 4.0 DRK seems to be designed to constantly use Dark Arts while having a very controlled amount of MP. Or you could make Scourge require a non-DA Syphon Strike to combo.2.) Scourge to become a third combo. One thing that bothered me about the changes from HW to SB, we have less to do. Managing DOTs has always been a pain for me, but if it was flashing in my face, it would be nice. Above my own problems tho, this would break the monotony of Soul Eater spam. Yes, it would be like PLD but it's better to be a copy of 3.X/4.X Pally, than boring 2.X Pally. I would also like for it to attain old Delirium's animation. I like it better than Scourge's, but that's just me.
I can agree with this, since then the skill would serve difference purposes with and without Dark Arts. The only concern I have with your suggestion is that you need the play around with MP consumption, because I'm under the impression Unleash was designed to be spammed, while Abyssal Drain was not.3.)Abyssal Drain to take the place of Unleash. Unleash is, animation wise, appealing but not necessary. Besides tagging mobs, its usefulness goes out the window, even more so with the nerf to its potency. If Scourge is to come back, I think it would be best to make that the level 56 skill, and AD made into the level 6 skill. I would like for AD to have Unleash's mp cost but I think I might just be crazy.
I think they're on the GCD to give you enough time between GCDs to pop Dark Arts and use their effects.5.) Bloodspiller and Quietus off the global cooldown. This is personal preference and not anything to do with gameplay. I've seen some people who want these to stay on the GCD but at least one of them off would be nice. If it's changed, I'll be happy. If not, oh well.
Eh...while I agree that DRK needs more defenses between 30 and 70, Dark Dance was only really useful when fighting multiple enemies. Reprisal was weird to begin with, because mitigation is supposed to be something you can count on, and its use relied on RNG. Low Blow was just lazy design (a stun with a resetable cooldown on RNG).6.) Anticipation, Low Blow and Reprisal to have the old DRK abilities added back. At the moment, DRK is only offering The Blackest Night as a great cooldown, but the old effects on reprisal, lb and ant were just more appealing. Dark Arts into Dark Dance, incoming reprisal and lb resets. It was a dream come true.
What I would suggest is Reprisal returning but as a filler ability that procs on parries (you parry, Reprisal becomes usable, and are able to do a 250 potency attack) to use between GCDs. Basically DRK's version of Shield Swipe.
You sort of missed the point of why the potencies were taken away. Utility skills like stuns and debuffs should not be used as GCD fillers for extra damage. For example, you can't design a boss around being stunned to interrupt certain moves if you have stuns with damage on every class, because then people learn to play by spamming their stun, leading to very early stun immunity and making the encounter impossible to clear.What I'm basically rambling is for the role actions, reprisal and low blow to get their potency back. I don't mind if they are 100 each, as long as I can feel that these moves are doing something.
I'll support 120s cooldown.7.) Shadow Wall cooldown lowered. It makes sense, and everyone is saying it. 90s would be amazing but if that's crazy talk, then I understand. I can see a 120 second cooldown.
On Dark Mind: This is sort of a relic from Heavensward at this point. I'm not a fan of specialized mitigation, but going along with what you're suggesting, I'd do this:
42 Dark Mind - Reduces magic vulnerability by 30%. Can only be executed when succumbing to the Darkside. Duration: 10s. Dark Arts effect: Redirects 10% of incoming magic damage back to the target. Cooldown: 60s.
This makes the mitigation on Dark Mind a little easier to use, while skill wanting to use it to mitigate bit magical hits and deal a bit of damage from it as a result. An alternative would be allowing magic damage mitigated by Dark Mind to empower a different attack (maybe Quietus or Bloodspiller?).