There are better ways to up play the Offensive potential of DRK. However an inverse HP skill in PVP would do more harm than good. It'd either be too strong or too weak. I can try to come up with something. However honestly Warrior is very much the DPS / offensive tank already. Just because DRK has utility doesn't mean it can't also be offensive. It's just not terribly good at anything right now and Warrior has offense covered so well already.
The easiest way to do what you're asking would be to make an augment to CnS making it a real threefold attack.(PVP) and giving them one more stun.
I'm not sure about fetter ward on DRK to be honest. It's an iffy thing but it could work well, though having a tank that can become immune to CC might be too much in mid - late game when the tank has to be killed if they have most of the medals, and if they fetter ward + run around > turtle > inulvnerable it may be too unbalanced.