3rd mage would be the simplest

shaman would bring a good amount of utility with totems, Mana Spring Totem/Totem of Wrath since everyones got a blue bar , tremor totem makes anything that fears easier and off healer in a pinch.

druid wouldnt be bad either nice dps, Gift of the Wild, earth and moon, faire fire, improved moonkin form, off healer

I would look into warlock also brings soulstone,Curse of the Elements, ability to summon which is really nice when yer farming and yeh notice all yer bags are full or need to repair :P, pet either felhunter for Fel Intelligence,Spell Lock and Devour Magic or imp for Fire Shield or Blood Pact

problay missed a few things :P