I tried the DH thing, if you’re coming from Frost you’ll like it if you’re coming from Unholy you may not, it’s a lot more…efficient. To the point it’s boring.
Rogue or Warlock or Spriest or Mage would be my suggestion. You’ll learn a lot more about the game playing something totally different and that way you get better overall instead of just improving along one line or style of play. I love my rogue because it forces me to play differently than I would on my DK. Can’t lock after all the butchering but if you’ve never tried one you wouldn’t know the difference. I don’t mean that to be snarky, I just don’t agree with the direction they went with Locks.