I've heard people consider a Warrior.
You'd have the intimidating shout, an extra buff, and another high damage burst class.
Charges for stuns.

A feral druid could be interesting.
The mangle-rake-mangle-mangle-rip does not need any facing requirements.
You get an extra buff, the equivalent of Kings (MotW) + Might.
Melee crits would heal the team, and you'd have a full blown healer + tank for PvE.

All four DK's have ranged attacks, in the form of Howling Blast.
Going with a straight melee, reduces your ranged attacks.
But gets you either a CC, a proc and/or buff, maybe higher burst.
3x HC and 3x DG, likely won't be that much worse than 4x of each... provided your additional class gets you something fairly big.