Disclaimer: I'm bored and not a DRK! :D
DRK's should get something like -ra elemental spells =O
Like how -AGA's became -AJA for BLM, remove drk from all Tier3 nukes and replace them with things like Stonra, Watera, Aera, Fira, Blizzara, Thundara.
With a low'ish MP cost (say 75% cost of a current Tier3 spell, like blizzard3 is 120mp atm, Blizzara could be ~85-90 making it just a little over Tier2 MP cost), reasonable damage (Tier3 on BLM sort of damage), low casting time (2.5~3.5'ish sec) and make them have an additional effect based on the nuke used, as long as you have a suitable Absorb-Spell learned aswell.
For example, let's say the DRK knows:
Absorb-STR
Absorb-DEX
Absorb-TP
Aspir II
Drain II
They then cast Fira -> Aera -> [some dark spell too whynot?!], would get an additional effect of "Absorb-STR" (giving you a STR bonus from casting Fira and reducing the mobs STR). Nothing from casting Aera, because this DRK doesn't have "Absorb-AGI" and maybe "Absorb-TP" or "Aspir II" from whatever Dark-based spell!
Also, for funsies!
Why not top it off with PLD/DRK onry summons? DRK could get Hades (on like a 30min recast or something), kind've like Bad Breath with Nightmare mixed in and high damage (though with a unique cost or something - like All your MP, leaving you at 1HP and generating a boatload of Enmity or something).