Some more ideas:
When water trickles into a crack in stone and freezes, the pressure from the ice makes it expand, damaging the stone. What would be neat is if an enemy is wet, you cast an freezing spell, and that damages the enemy's armour, making them easier to kill.
For sneaking (sneaking should be in the game, right):
A rain spell to put out torches and such outside, and a wind spell to put out lights inside (or outside), so you can put out lights. Obviously, there'd have to be a chance for enemies to detect or not suspicious activity.
I'd like to see a freezing spell that is cast in an arc around the character, so if the caster holds position, enemies that charge have a chance to slip and fall.
I think the Summon Rat spell from Divine Divinity should make a comeback as well. It could be moved by the player (while the player is controlling it, they can't move themselves) and used to check for traps, or to squeak on command to distract guards for a moment. This time, it really IS "just rats".
Spell combinations:
Rain + Earth = mud that slows movement and grants a chance to fall.
Rain + Ice + Air = Blast of hail
Rain + Fire + Air = Concealing steam(?) (Probably not that great an idea.)
