Eh, Dark Knight isn't that flawed... Parry is the only real issue as far as core flaws go, but that goes for all tanks and is mostly because SE got confused and gave Warrior Raw Intuition for some reason... Paladin gets Sheltron to trigger Shield Swipe, Dark Knight doesn't really have anything for Reprisal... MP in PvP is the biggest issue, simply due to how the Jobs is built for PvE; Blood Price and Blood Weapon are far more effective against static AI, thus Dark Knight loses some of its MP in PvP. Furthermore, PvP is built around burst, while Dark Arts was not designed with that in mind... That's a very easy fix though, while Parry needs a complete overhaul IMO...
There are plenty of minor changes I'd like to see them implement to Dark Knight in general (some of which are almost certainly stupidly unbalanced), but as far as PvP goes they can entirely "fix" it with PvP skills... They can adjust the existing PvP skills to provide more stopping power, and beyond that the only thing Dark Knight is really missing is a form of party healing... Warrior gets that from a PvP skill, so can Dark Knight... As far as damage and burst goes, the Job is perfectly fine provided you know what you're doing with MP... Grit is perhaps the only other thing worth moaning about, but that's tied to MP as well so I wont bother... Addressing MP issues not only fixes a lot of Dark Knights problems, but it also lowers the skill level required to play it effectively, that's perhaps the single biggest adjustment they can make to the Job right now...