The Weather Warfare
I think there's not many Strategic Spells that affect the map in non-direct destructive way. Would be great to add couple of spells that control weather. Let's say you could conjure a blizzard which slows down the march of the armies / caravans, burning sun could couse a shorteges in food, floods could destroy buildings etc. Of course there should be a way of applying counter-spells, but i think that would be great (and annoying for enemy) addition to this already nice game.