If my memory is good, in another mmo (WoW I think) there is a tank with very short Cds.
Sounds cool to me: imagine if TBN was a 5s duration Cds like now, but with an short recovery like 10s, with a little cost of mana, like... The half of now.
I think this should fix the problem of the DRK's mitigation, since you can using it more often with a dps boost.
I just saying this like that, without any maths, but just an idea.