Imo, I'm fine if they give it a greatsword and make it a DPS class. Sadly, this game is not using a dual/multi-weapon system, so we gotta pick ONE type of weapon for them.

My idea of a DK is: Active buffs that drain your HP to increase the damage (The similar way the bard's songs drain MP to buff you) or sacrificing a fixed amount of HP to deliver special attacks. Getting "Blood" stacks can also work, using certain HP draining skills, it gives you stacks like the monk's combos. Also: Debuffer. AoE damage.