Wouldnt the combination of Marauder and Summoner fit the Dark Knight nicely?
When you look at FF XI the DRK was using Greataxes or Scythes and used spells like Bio, Energy Drain and stuff. The Marauder even has Bloodbath ready for use. Its almost to perfect a fit.
Tho i'd love it, if they would change the weapon from Greataxes to Scythes when you equip the Soulcrystal, but thats just my opinion.
They just have to give the DRK a "Costs health and deals more damage"-Ability when you unlock it on 30 and your good to go.

Concerning the RDM mentioned a few times in this thread.
I'd like to have him like in FF XI too. More of a Support-Dps with EN-Spells and Debuffs like Poison and Paralyze, who excels in weakening the enemy.
Would be nice if he could "weave" his spells into his attacks trough procs or something.

PS.: I want my wyvern back