personally id like drk to get three things in the future.

1. an on demand self heal
Pld has clemency and wars designed around self heals so that's a given but drk self healing is either something you cant use when you need or is too weak to heal you enough to waste mp for, yes abyssal drain is good but only when you've got multiple adds on you as such id like there to be something i the case you need hp (like living dead for example).
best thing i can think of is either a new skill for it or modify unmend to be DA'able to drain and get back a more significant amount of health.

2. Utility
at the moment drk really lacks anything impactfull to the raid (reprisal being useless unless you take dmg or delirium which is useless if you have a monk)
as for what to do for it, thats tough though ive thought id be pretty fun if drk could boost
dot damage for the raid and from there alter delirium to either stack with dragon kick or even allow drk to keep up dragon kicks full effect once it has been applied by the monk and finally allow reprisal to be dark artsable to activate, possibly at an mp cost unless it procs.

3. another dark arts skill
i think one problem ive had with dark arts is i always feel its more useful to use for offensive purposes (not saying not to use it for defensive skills but its something at times if you wanna keep dps up, might just be the way ive been with the job)
one way i was thinking was to add a second one which can only be used every 20 seconds at half the cost, could even add a future skill that requires two dark arts stacks to activate making the skill.

of course theres alot of ways square could go with it, most of all id just like to see all the tanks be able to ot or mt in the future.