Relates to this idea (monster spawn dependent on terrain type):

https://forums.elementalgame.com/388203

And ideas for elemental resistance here:

https://forums.elementalgame.com/388176

It should be possible, even desirable, for an ice wizard player, for example, to change his territory into an icy tundra.  This should give him a tactical advantage, since he specializes in ice magic, researches gear and spells that protect his units against ice, and has icy monsters spawn near his territory.  To assault this player, opponents would need fire magic and/or ice resistance--or else suffer heavy casualties.

Earth, Air, and Electricity aren't so easy.  Earth may be rocky or heavily forested terrain and golems.  Air could be birds?  Not sure what terrain would represent air or electricity.  Maybe electricity is semi-permanent storm clouds--blinding and randomly damaging the enemy.

This might also be good mod material.  A mod could focus on just ice magic/terrain/creatures.  Over time, the mods for each element could be combined into one mega mod--or expansion pack.