To be honest, I would wish to see Dark Arts become a more central part of the job's kit: right now, you only make use of it when breaking TBN, and the more you do that, the less MP you have to spend during raid buffs.

What if Dark Arts made the next Edge / Flood of Shadow heal you for the same potency as damage dealt?
What if they made the free EoS/FoS have 15% increased potency also?

Many people go on about it having to be dps neutral, but why exactly? I have done all the ultimates except TOP, and several savage floors and cannot really imagine a situation in which you'd take a damage down to get one free Dark Arts. In casual content, why would you really care about a tank having 4-5 vulnerability stacks? That content barely hits hard, you have a load of defensive CDs and granted you wipe once the DRK should know his limit.
That content also does not really require any damage optimization, if anything its completable with you mostly messing up your rotation.