If UBRS is your goal to master, then melee is the way to go due to the high magic resists of the dragon type monsters.
If you want to keep your mages as main dps, you will need a warlock for the elemental curse (-75 resist and +10% damage).

Rogues/Feral druids are hard to control due to the combo system (they will always be out of sync in terms of points).
So simply take 2-3 warrior dps' and replace one of the priests with a paladin; the buffs are just too strong to miss out.