I'm going to say this from a healer's perspective. I believe DRK is balanced within the context of the Trinity System and that the other tanks are currently outside the scope of balance.

Having said that, DRK does suffer from a lack of damage and mitigation in comparison to its peers, so I'd be fine with increasing those aspects of its toolkits.
For Dark Mind and Dark Missionary, let's add on a buff that increases the amount of healing received for people under the effects of these buffs.
Add back Shadowskin as a low level version of Oblation that provides only 10% mitigation and upgrade into Oblation that grants a 2nd charge and increases the mitigation effect to 15%. Reduce the CD of both to 45s.
Reintroduce Dark Passenger and Scourge as proc upgrades for Flood and Edge of Shadow when TBN breaks.
Decouple Carve and Spit and Abyssal Drain.