I've been thinking about adding a tank to the mix, and 5 boxing just for the sheer fun of it. PvE is 100x more enjoyable and really getting into it with this expansion. I think it is highly due to the encounters being more intriguing and that tanking got a LOT easier.

The biggest bang for your buck is an Unholy DK because they add 13% damage to the group.
If you prefer a different tank, you can drop a Shaman and place a Boomkin which some have used with great success.