So I think one of the areas of improvement needed for DRK is their lack of decent raid utility. Obviously, just give them some type of slash down debuff would mostly resolve that problem but I don't really think it's the best way to go since that route is already covered by two other jobs.
So here is my idea: change the damage type of DRKs from slashing to magical damage. This would open up for some pretty interesting job synergy that the game is currently lacking.
First, while I understand it make sense that the massive swords DRKs swing around do slashing damage, however as we know DRKs are plenty familiar with the magic side of things. I don't think it's too much of a leap of logic for the lore enthusiasts to accept that DRKs extend a magic aura around their blade or whatever which would covert the damage type to magic.
Now if we do this, it opens up an interesting opportunity for some unique raid synergy. In addition to changing the damage type, a -10% magic resistance down debuff should be added either to Power Slash or Soul Eater (that doesn't required DA to activate). This would give a nice little boost to DRKs to make up for the lack of a slashing debuff of their own as well as provide a unique extra boost to the other half of the damage dealers in this game.
Right now we don't have any melee skills that help out our magic damage friends. Foe's and Bishop's Turret pretty much are the only two skills in the game that cover this. I think it would be great to see a tank that is focused on improving the magic capabilities of a party and it seems like DRK could be made to be it.
Now I don't claim this is to be the ideal solution when balancing come into play. I'm aware that there are three different kinds of melee dmg but just one kind of magic dmg so I just don't have enough damage data to really know if this will completely throw off the dmg output of a party as desired by the developers. Of course there are ways around this still if a 10% increase on top of what already is available is too much. You could for example drop it to 5% and have Foe's overwrite it if 15% or 20% is too much to stack but I'll leave those kinds of details to the developers.