A horde of devious, ferocious, tenacious, and atrocious creatures, beasts, and monstrosities to populate the northern realms of your fantasy. Get them while they're cold!
Perhaps those that practice elemental magic begin to take on characteristics associated with their chosen element. For instance, an earth elementalist might be prone to agoraphobia, while air and possibly fire elementalists might have problems with claustrophobia. Water elementalists might always seek the path of least resistance. A fire elementalist might have a cat's opinion of water. This could also apply to physical differences. Fire elementalists might have a freakishly high metabolism and a permanently high body temperature. Water elementalists would probably never get dehydrated, but might slow down a lot when it's cold. Etc, etc.