I think Stabbey's original idea of limiting elemental summoning to their respective terrain would fit the game perfectly, given that it's already very strong in the terrain interaction department.
On the other hand I do see Seelenernter's point. At the very beginning summons seem strong and you can cheese some fights with them as long as they have to hold off their own element. Like you can block a passage with a fire elemental against fire creatures. Later on however, in fights not bound to a certain element, you often have better things to do with 6-7 action points. A buff or a debuff will last 3 turns but your summon most assuredly won't. Besides, its elemental attacks won't be of much use against late game enemies.
Maybe summons could scale with level or intelligence to become more potent?
