I don't have time to really flesh this out at the moment, but a stray thought:

Instead of Dark Arts being a per-use ability, it could give you 3-5 'Stacks' of Dark Arts (stackable up to 10 so you can still burn excess MP on it), which are then automatically consumed one-by-one per weaponskill to add 30-50 potency to that weaponskill (like Machinist ammo in 3.x), and are also consumed by DA-compatible oGCDs like Dark Mind, Dark Passenger, and Carve and Spit.

Pros: less reliance on double-weaving, Plunge is usable between more weapon skills, gives DRK back its consistent high damage identity from 3.x rather than pseudo-burst, allows skills like Dark Mind to consume less potency than a full Dark Arts 'cast'.

Addendum: Delirium: Dark Arts increases weaponskill damage by 150 potency instead of 50 for 20 seconds.