It would seem to me the solution to the ice mage/fire mage is not in changing the monsters, but in changing the spells. Maybe the ice mages from the north have spells, especially dailies, that have a 'resistence effect'. They Slow the opponent in addition to the lessened damage. Maybe the fire spells of the desert nomads flare up on the skin of the resistant, dazing them. It would seem to be a faster fix than changing the effects of the keywords.