My personal issue with DRK is simply the way it plays. Once you have initial agro in fights, you are left spamming souleater. Now these are just some thoughts about changes to make the play more interesting. I have no idea about exact damage numbers, simply listing some ideas...

Scourge brought back to add as a second combo ender for syphon strike. Dark arts gives lifesteal:
This would help break up the monotony of doing souleater over and over.

Plunge removed potency and reduced cd to 15s gives high amount of enmity. Too many times I am holding this skill back for use on knockbacks:
If we arent going to get some kind of knockback immunity then this would help a lot. As the skill would be up when we need it

Dark passenger has the mp cost removed or potency buffed:
I just dont really find myself using this skill much and with the loss of plunge as an ogcd would probably be a good idea to add this to the single target rotation.

Sole survivor becomes a damage over time on a much lower cd but still has the leach on death effect but potency reduced:
This is such a cool looking skill and it really kills me not being able to use it very often.