Adding a Warrior to DK's adds a fair bit for PvP, at the cost of fewer things to round-robin with the DK's.
I know your thread is for PvE content.

If you want to drive a warrior, there's nothing to stop you from using one as your tank.
You get the extra shout, for health but I'm not sure if it will stack with Kings or not.
But then you could go with the Commanding Shout + Blessing of Might.

You could add a Priest to the mix too, if you like.
Then your Pally could be a Ret, or you could replace them.
More of a mix of classes is not as much of a detriment for PvE focus.

The Priest character can be a Healer or a Caster DPS.
The Paladin character can be a Healer, a Tank or Melee based DPS.
The DK's can be DPS or Tanks.
The Warrior is DPS or a Tank.

So decide who you want to heal the team with.
The Priest gets you Shackle (crowd control, vs Undead), AoE fear, Shield, strong AoE Heals, is a fairly strong mobile healer, Fortitude is a pretty strong buff.
The Paladin is a superb single target healer, will passively heal themself through healing others plus one other toon with Beacon; they have either Kings or Might, which is a strong buff.

Then decide who you want to tank with.
The warrior is demands more micromanagement, but that's not saying they cannot work and work well.
It is a preference thing really.
Even if you won't want the warrior as a tank, you might want one on the team to add diversity.

Lots of choices.