How would giving DRK "Bloodbath effect on EVERY defensive cooldown" necessarily "diversify DRK's defensive CDs"?
To make it feel more like Dark Knight.
But again, why would that necessarily be "more like Dark Knight"? Dark Knight has historically had HP-spending actions about as often as HP-generating ones. Moreover, in XIV, Warrior has already been the 'live by the sword, die by the sword' healing-rather-than-mitigation job that survives through damage dealt. While that, like DRK's initial theme with more versatility (or, choicefully allocated resources / chosen spenders), has been watered down over time, it's still probably the more natural fit given either job's identities as established here thus far.
Forcing me to be redundant, I see...
Only to be clear, preferably.
would it at LEAST okay to rework Dark Arts to be similar to at least 1 Dragoon's Dragon Gauge mechanics...?
Which gauge, and to what purpose? Scales? Eyes? What mechanic is it supposed to fund / be funded by?
Dark Arts is currently just a fancy name for the feature that allows TBN to be more flexibly timed than the other on-demand CDs. Will you be leaving that feature as is when taking the name for something else (such as each Shadowbringer requiring two "Eyes/Scales" --each generated from another ability-- for it to be cast, or each granting an "Eye/Scale" to be spent on some other/further ability)?
Moreover, what is your intent in reducing DRK's opener and only its opener (not any burst cycle thereafter)? Such mechanics already pretty uniquely mess with the opening rDPS of DRG (via LotD not being available for the opener) and the Physical Ranged DPS; would this be intended to indirectly nerf the contribution of potions and to make it more normal not to have a kit's full strength available in the opener? What's the intent here?
Dark Mind would be an upgrade for Dark Dance...
Then you'd be replacing what you've specified as a 10-30% (average: 20%) physical mitigation + 10% magical mitigation skill... with a 20% magical mitigation (+ maybe 10% physical mitigation?) skill on the same CD... Why?
Well it's not like you going to cheese a lot of mechanics to begin within those 20 seconds
I don't have the time right now to go through each fight to show you where 20-second invulns would offer additional free mitigation, nor is it uniquely breaking fights or the like the issue. The problem is that the more powerful they are, the more they devalue the rest of the kit -- and far more so if allowed such absurd (i.e., double and already overlong) duration.
Let's get the "why AoE AND cone" out of the way first... More AoE damage is good for pulling in dungeons in general, also I believe the cone covers the targets you are trying to hit better as opposed to a circle, but then you gotta factor server ticks for both cones and circles and that is too much to figure out on my own...
The effect of server ticks isn't going to vary between circles and cones except in that, at similar total square area, the first will have lower range but will feel more lenient. In general, a circle will probably tend to be a bit more intuitive for a gap-closer because you generally tend to use the gap-closer to get distance, not just as a short dash to the outermost enemy from which to fit all enemies into the cone. The latter tends to work better for after you've at least face-pulled, since then you can gather the enemies and potentially jump towards the back of the group, aiming the cone backwards into the more gathered enemies. A circle, on the other hand, allows you to jump straight to the center of the pack. Neither is inherently better, but the cone is more likely to appear to "ghost", failing to hit enemies that the animation crossed over because of how far apart the server-side unit positions are from how their portrayed to the player.
To be clear, given the same units-squared of area either way, I'm more a fan of cones that circles for most AoEs. I just think you'd end up with very frequent "ghosting" if you attached a cone to a gap-closer, and generally frustrating players more for their being cones than if you attached a circle AoE, especially if you don't shift the cone's point of origin to some 2-4 yalms behind the player (at which point, you lose much of the extra range anyways).
All that being said, I truly don't think it'd matter. The main change you're going to face in strapping an AoE to a gap-closer is that in dungeons / add spawns you then make it less a gap-closer than a source of AoE damage, which can make it feel less responsive. In short, you'd trade QoL for one little thing more to optimize, which you'll then likely be balanced around (meaning you're at the same output ceiling, but nerfed for not optimizing).