I'm aware that DRK is having viability struggles but I think there is a bigger issue.

Problem: After stormblood took away warrior's enmity management and dark knight's dark arts they lost their identities. Warrior found a new identity after the Nascent Flash buff and has been going strong since then, the devs still don't know what to do with DRK

Potential Solution: Make DRK the opposite of PLD, make it the black magic tank. Make it's burst end with a long cast. Remove plunge and replace it with something like Icarus. Give them a dot spell they have to keep up, like poison.