Locks dots aren't as efficient in ym experience in groups with lots of burst, which you'd have with the mage and either a shaman or boomkin. I'd say go shammy and boomkin for your other 2 dps, they can both heal, totems are good in caster heavy groups (or melee heavy, not as good in mixed with only one shammy) and the druid bring battle rezz, buffs and scouting ability with prowl, and can offtank in a pinch in bear form.