I am definitely wanting to run with a resto druid. Symbiosis on a Spreist nets the comp a second use of Tranquility as well as a second Life Grip. With spreists being so darn hard to kill alone adding druid Hots to the mix makes it very hard to train us. Spriest and Druid mobility and all the peels will give a nice edge against melee teams. I'm thinking an unholy DK to pair with it to help give additional edge against caster comps (strang, mind freeze, grip, pet stuns, amazing AMS will drive casters batty). A DK speccing into Purgatory can help take the danger off of using Voidshift at too low hp. If it drops to 20% and gets switched on, between spriest off heals and the rdru it won't be hard to save him. Deathgrip and chains are also more great peels making the comp very very mobile. The druid can also Sym the DK against TSG or other heavy stun teams in order to gain Icebound Fortitude and stun immunity.
Another idea I was considering is Spriest/RDru/Hunter. A BM hunter can put out absolutely insane amount of burst damage while having a great array of control options to help set up kills. Silencing shot, scatter trap, monkey stuns, disengage's root. The hunter also brings freedom to help peel even more. Sym on hunter gives the RDru deterrance and the Hunter another root break.