I have an idea that's a bit more..... out there, per say, but it came into mind because of the main conversation you've all been sharing.

Keep in mind this is all pure theorycrafting at this point too lol

So to start, what if Darkside instead of giving bonus damage it instead granted haste, similar to the NIN's Huton, and the DRK got a new combo finisher that gave Darkside, meaning it would have to rotate between healing itself and getting Darkside. Ofc then cut most of the oGCD abilities entirely, with a couple exceptions I'd imagine, but instead of being a basic combo tank with fuckloads of oGCDs to make the job feel "busy" it would instead be the low GCD tank that was actively fast paced. Then we'd focus on the GCD rotations with "modes" we could rotate through, maybe toggles or something akin to the BLMs Astral and Umbral modes, if not just duration/charges. These would be oGCDs ofc and they'd be something like the old Dark Arts, where you'd drain MP while empowering your GCD rotation, giving them extra effects and not just bonus potencies (maybe even unique animations... I can dream... lol), and then another one that was just used for MP regen. There could also be multiple kinds of Dark Arts, where one gives bonus damage through DoTs or just extra attacks/damage, and then another one used for sustain where it gives different sustaining tools like cures and HoTs, even shields. So you'd basically go into a Dark Arts rotation then go through an MP regen phase and you'd have to pick which version of Dark Arts to go through since you'd only have the MP for one of them. If they're toggles you could even actively swap between all the modes, being forced into the MP regen phase once you ran out.

Then ofc remove the MP for TBN and make it a beefier shield based anti-buster to match the other three tanks, but all the defensives could be kept in their numerous versions as they'd serve as our main weavable abilities with the reduced GCD from Darkside.

This would differentiate the DRK from the WAR as well as make sure it didn't just become another GNB with all it's oGCDs. You might also think "that would just get boring, mostly just having fast combos," but the NIN is certainly far from boring and it essentially is just fast GCDs and Ninjutsus. So rotating through different Dark Arts would be the variations in our play patterns.

If that's not enough add in a new "Delirium" mode where you have to go through a set number of Dark Arts rotations or uses and you'd get access to a "super mode" that gave one "dopamine" rotation of just three flashy big hitters that also worked as a quicker MP regen phase, or something like that lol

This is all just farting out of my head, full theorycrafting, but I am nonetheless curious if this would be something along the lines of what people want? Or if I'm just lost in fantasy land at this point lol