Is it sci-fi or fantasy?
Don't forget your undead! You could use the same principle of the summoned elementals, but instead use the ol' standby...necromancy! A necromancer controlling anything from a small squad to a large group of undead beasties. Nice! By doing this along with an elemental line of magic, you'd be tying magical abilities into some sort of commonly-threaded system.
Also, you could refer to the old movie, Fire & Ice. You'd have 'Necron', or a powerful mage able to control the elements and a horde of subservient subhumans. Opposing them would be the forces that use fire and lava, but are otherwise human and very medieval in structure. Oh yeah, they fly large pterodactyls, which is always cool!